THE PHYLOGENETIC RELATIONSHIPS OF THE SPECIES IN
THE DROSOPHZLA MELANZCA GROUP
HARRISON D. STALKER]
Department of Zoology, Washington University, St. Louis, Missouri
Received October 4, 1965
HE cytological, taxonomic and genetic relationships of the species belonging
to the Drosophila melunicu group have been investigated by a number of
workers, and the accumulated information has now reached the point where
summing-up seems desirable, with a view to describing the phylogenetic relationships of the members of the group as a whole.
The taxonomy, geographical distributions, and genetic studies may be summarized as follows (see PATTERSON
and STONE1952; STALKER
1960, 1964a, b,
1965; THROCKMORTON
1962, for further references). The group belongs to the
subgenus Drosophila, and on the basis of morphology, is considered to be fairly
close to the D. robusta and D. unnulimnu species groups. There are seven Nearctic members of the group, of which one, D.melunissimu, has not been reared in
the laboratory, has not been studied genetically, and will not be considered further
in this paper. The other six members are: D. micromelunicu, D.nigromelunicu,
D. melanuru, D. euronotus, D. puramelanicu and D. melunicu.Of these six species,
five show clear northern or southern distribution patterns, although there is a
good deal of overlap. All species reach the Atlantic Seaboard states in the east;
the remainder of the distribution is summarized below.
The southern species: D. micromelunicu, disjunct distribution west to Arizona,
north-south range 25" to 37" N lat. D. euronotus, west to eastern Kansas, northsouth range 26" to 42" N lat. D. melunicu, west to northwest Arizona, northsouth range 26" to 41 ' N lat.; this otherwise southern species also has one or more
apparently isolated populations in northern Montana.
The northern species: D. melunuru, disjunct distribution west to eastern Oregon, north-south range 34" to 48" N lat. D. parumelunicu, west to eastern
Nebraska, north-south range 33" to 49" N lat.
In the case of D. nigromlunicu, the species extends west to eastern Oklahoma,
and has a north-south range of 28" to 47" N lat. and thus cannot properly be
classified as either northern or southern.
Interspecific crosses have been attempted in the six species which can be cultured, and the results are summarized in Figure l. Details and references for
these tests may be obtained from the papers of STALKER
cited above. It will be
noted that while D. micromelunicu will cross with no other species, the four
species. D. melunuru, D. euronotus, P. purumelunicu and D. melunicu will cross
Thls research was aided by grants from the Natlonal Sclence Foundation
Genetics 53: 327-342 February 1966.
328
H. D. STALKER
D. MICROMEL6iiICA
D. NICROMELAIiICA
D. MELAWJRA t---D. PARAMELAMICA
-
1 -X T1
D. WROMOT[IS
D. M E W I C A
FIGURE
1.-Summary of hybridizations reported in the D.melanica group. I n every case the
arrsw points from the female to the male parent. GRIFFEN(1942), in reporting hybridization
involving the species D.nigromelanica, D.paramelanica and D.melanica reports all hybrids to
be fertile in both sexes. Other workers have found hybrids to be female fertile but male sterile.
in certain combinations to form fertile hybrids. It has been the general experience
of workers on this group that all hybrids formed are female-fertile but malesterile. GRIFFEN’S(1942) report that the hybrids of crosses involving D. melanica
and D. paramelanica are fertile in both sexes could not be confirmed. GRIFFEN
has also reported that he obtained hybrids in the crosses: D. nigromelanica
females x D. paramelanica males and the reciprocal, and indicates that these
hybrids were also fertile in both sexes. The unexpected nature of this report is
discussed by STALKER(1964b), who has been unable to obtain such hybrids.
There is no evidence of hybridization between any D. melanica group species
in nature.
On the basis of both hybridization tests and morphology, the members of the
group may be divided into three subgroups, the subgroups including D. micromelanica in the first, D. nigromelanica in the second, and the other four species
in the third. As will be shown below, such subgrouping receives additional phylogenetic support from the study of interspecific differences in the salivary gland
chromosomes.
and NOVITSKI(1941 ) showed that two reIn D. micromelanica, STURTEVANT
productively isolated subspecies existed, one in Texas, the other in Arizona.
STALKER
( 1965), working with the material then available, found two subspecies,
the Eastern one in Virgina, Florida and Texas, and the Western one in Arizona.
Crosses within subspecies yielded fertile progeny of both sexes, crosses of Eastern
females by Western males produced fertile female but sterile male progeny. The
reciprocal crosses yielded fertile offspring of both sexes.
In all of the six species except D. micromelanica, the metaphase chromosome
pattern is essentially the same. There are in females: a large pair of V-shaped
X chromosomes, a pair of short V-shaped autosomes, two pairs of long rod-shaped
autosomes, and a pair of dots. In D. micromelanica a variety of karyotypes have
been described (see STALKER
1965 f o r discussion), but they all differ from the
pattern described above in having a rod-shaped X chromosome, and as expected,
an additional pair of long rod-shaped autosomes.
Since, f o r theoretical reasons, fusion of chromosome arms is expected to occur,
and has in fact been repeatedly observed in Drosophila speciation, while separation of V-shaped chromosomes into rods is unlikely and has rarely been observed,
it may be assumed that the rod-shaped X of some ancestral form like D. micromelanica has fused with one of the autosomes to form the V-shaped X found in
Drosophila melanica GROUP
PHYLOGENY
329
the other five species. This assumption of fusion rather than separation is especially applicable to the case of association involving the X chromosome, since as
WHITE (1964) has pointed out, when autosomes become included in the sex
chromosome (as the result of X-autosome fusions), the autosomal arm may be
expected to acquire sex-determining properties such that later separation of the
two arms would result in an unworkable sex-determining mechanism. In summary, then, it is assumed in this paper that D. micromelunica is more nearly
ancestral than any of the other five group members.
The structure and variability of the salivary gland chromosomes of D. melanica
have been studied by WARD(1952). The other five species have been studied
by the present author. All species showed intraspecific chromosomal polymorphism and a total of 80 intraspecific inversions have been found to date. Although
inversion variability has been found in all chromosomes except the dot, the
inversions are very unevenly distributed, with certain chromosomes such as
the second autosome and the X relatively variable, and the other autosomes
relatively conservative.
In working out the constant (homozygous) interspecific chromosome differences, any hybrid larvae which could be produced were of course extremely useful in demonstrating the nature of the species differences in banding pattern. In
the numerous cases in which no species hybrids were available, or in which the
hybrid salivary chromosomes were of inferior quality, the species comparisons
were based on a study of photomaps of the standard chromosome banding patterns of the various species. Using such photomaps, it was possible to find chromosome regions showing the same banding pattern in the species being compared,
and by using such homologous regions, to work out the series of interspecific
inversions that would be required to convert the gene sequence of one species
into that of another. In this work, the photomaps were used as book-keeping
devices, rather than as primary evidence of homologies. Evidence of homologies
was based primarily on comparisons of a rather large collection of supplementary
photographs. The advantages and disadvantages of this photo-comparison method,
and the techniques involved in constructing the photomaps are discussed in
STALKER
(1965).
Before any attempts were made to work out interspecific banding differences,
all available intraspecific inversions were mapped, and where possible, related
phylogenetically. Using such intraspecific phylogenies it was frequently possible
to guess which of the various gene sequences within a species might be most
primitive (usually on the basis of its central position in the phylogeny), and
initial interspecific comparisons were made using such presumably primitive
sequences.
In the account which follows, two nomenclatorial modifications are introduced
in the interests of simplicity and clarity. First, the chromosome arm which is
labelled X-left in all species except D. micromelunicu (in which it is an autosome)
will be labelled “X-left” or “XL” in that species. Secondly, a new system of
letter symbols representing inversions will be used. The original inversion lettering systems were worked out on an intraspecific basis, with such symbols as A,
330
H . D. STALKER
B, or C representing quite different inversions in the different species. In this
paper a single system of letters will be used for each chromosome element, starting with f (Standard) in D.micrornelanica, followed by A, B, C, etc., and when
the alphabet is used up, symbols such as A’, B’ and A”, B”, etc. will be employed.
In this system an inversion such as A bears no special relationship to A’ or A”.
The symbols used in this paper are the same as those used by STALKER(1965)
in the discussion of D.micromelanica, D.nigromelanica and D.melanura, but
differ from those used earlier by WARD
in D.melanica and by STALKER
in D.paramelanica and D.euronotus. I n describing the chromosome phylogenies for the
group as a whole, the assumption is made that the gene sequences in D.micrornelanica are ancestral to those found in the remaining species. In the diagrams
(Standard sequence) will be
which follow (Figures 2 through 5 ) , the symbol
used only once f o r each chromosome, and will always represent the arbitrary
standard sequence in D.micromelanica The various gene sequences found within
any given species are enclosed in a box, the arrows leading from one box to the
next may carry letters which indicate the number and designation of the interspecific inversions, and percentages which indicate the proportion of the chromosome which could be homologized in that particular interspecific step.
+
RESULTS
The phylogenetic changes in chromosome 3: As indicated in Figure 2, chromosome 3 shows two sequences in D.micromelanica (+ and A). I n all of the other
five species, inversion C is found. In the phylogenetic branch leading to D.nigro-
1“.
- 1 1 1 1 1 1 1 1 . 1 . 1 1 1 1 1 1 1 1 l l l l l l l
!
U.
I
MICROMELANICA
STANDARD +
!
,,-,-.a
!lll.l,ll
1 -l
1 1lll-,l
C
Hypothetical
ylll.~llllllll.~..
I
1 , 1 . 1 , 1 1 1 , 1 1 1 1 1 .
1
D. MELANURA
! D.
! D.
PARAMEUICA
MELANICA
CDE
I1.lll.l,l.l.l,l,l,l.l.l.l.~
I
FIGURE
2.-The phylogeny based on inversions i n chromosome 3 in the D.melanica group.
The letters inside the boxes represent the heterozygous or intraspecific inversions, letters between boxes represent homozygous or interspecific inversions. For example, in D.micromelanb
the third chromosome may show either the Standard (+) arrangement, or the inverted A arrangement. The two species D.melanura and D.euronotus have a single common sequence CD,
which differs from the Standard of D. micromelanica by the homozygous inversions C and D.
The percentages indicate the proportion of the chromosome which could be homologized in any
particular interspecific step.
Drosophila melanica
GROUP PHYLOGENY
33 1
melanica, an additional interspecific inversion B leads to the species standard and
only known sequence for D. nigromelanica, BC. In the other branch, addition
of the interspecific inversion D to C leads to the CD sequence, which is the
standard, and only known sequence in the two species D. melanura and D.
euronotus. Proceeding from these species, addition of the interspecific inversion E
leads to the CDE sequence which is the only one found in the two species D. paramelanica and D. melanica. It will be noted that in all three interspecific steps
it was possible to homologize 100% of chromosome 3 between the two species.
If it is granted that the sequence starts with D. micromelanica, then the third
chromosome indicates a branched phylogeny, leading to D. nigromelanica on the
one hand, and the other four species on the other, with the two species D. paramelanica and D. melanica more recently derived than D. melanura and D.
euronotus.
T h e phylogenetic changes in chromosome XL: In this chromosome (Figure 3),
a single sequence occurs in the “XL” of D. micromelanica; addition of the two
inversions A and B leads to the single known sequence AB in D. nigromelanica,
and a different branch leads (with the addition of interspecific inversions C, D
and F) to the sequence CDF in D. melanura. Addition of the intraspecific inver-
i
D. M E L A N I C A
I
i
I1
II
II
I
CDEFGHM
.”,
t 1I 1
.
1
I1
I1
I 1
I 1I 1I
!
..
I1
1
,
I
FIGURE3.-The phylogeny based on inversions in chromosome element “XL,” in D. micromelanica and XL in the rest of the D.melanica group. The general meaning of the symbols is
as in Figure 2, however specific symbols such as A or B in this figure have no relationship to
the same symbols in other figures. In the box f o r D.paramelanica, the symbols JKL and I represent chromosome sequences which also include the inversions CDEFGH. Thus in D.paramelanica
three different XL sequences occur: CDEFGH, CDEFGH-JKL and CDEFGH-I. The symbol N
in D. melanica should be interpreted in a similar fashion.
332
FIGURE
4.-The phylogeny based on inversions in chromosome 2 of the D. melanica group.
The general meaning of the symbols is as in Figures 2 and 3. Those intraspecific inversions of
D. melanica which are located on the proximal two thirds of the second chromosome are not
included in this figure, since that portion of the D. melanica chromosome was not homologized
with chromosome 2 of D.paramelanica. See text.
sion E in D.melanuru leads to the alternate D.me2anuru sequence CDEF, which,
with the addition of the interspecific inversion G leads to the only known sequence (CDEFG) found in D.euronotus. Continuing the analysis in this way
through to D.melunicu, the complete X-left chromosome analysis leads to a phylogeny which, starting with D.micromelanica, gives off D.nigromelunicu on one
branch, and D. melanuru, D.euronotus, D.puramelanica and D. melanicu, in
that order, on the other branch. Thus the phylogenetic sequence based on X-left
agrees with that based on chromosome 3.
The phylogenetic changes in chromosome 2: Chromosome 2 is the most polymorphic in the species group ( Figure 4), whether one considers intraspecific or
interspecific inversions. In this chromosome a total of 56 intraspecific inversions
have been described, and it is known that at least 20 interspecific inversions exist,
although since the second-chromosome differences between D.puramelanica and
Drosophila melanica GROUP
PHYLOGENY
333
D. melanica have been only partially analyzed, the actual number of interspecific
inversions is certainly higher than 20, and is estimated at 25.
Examination of Figure 4 will indicate that a number of rather unusual features exist in the phylogeny. First, despite the very large number of intraspecific
inversions, and the rather extensive intraspecific inversion phylogenies, the pathway t h o u g h these intraspecific phylogenies is frequently a remarkably simple
and direct one. This is strikingly illustrated in the case of D.euronotus. The
initial sequence in the D.euronotus intraspecific phylogeny is WXYZA’B’C’E’
H’J’. The intraspecific sequence which leads out of the intraspecific phylogeny
towards D. puramelanica differs from the initial sequence only by the addition
of the single inversion V’, thus in the pathway from D. melanura through
D. euronotus to D. paramelanica, almost all of the extensive D.euronotus intraspecific phylogeny is by-passed. A somewhat similar situation exists in the passage
through D.paramelanica to D.melanica. Here if we consider only the distal third
of the chromosome arm, since the proximal two thirds was not analyzed between
D. paramelanica and D. melanica, we find that the J”, J”M”, J”K”, J”L”,
J”K”L” part of the D. paramelanica phylogeny is by-passed in the passage
through D. paramelanica. A somewhat comparable situation obtains in the
branches leading from D.micromelanica to D. nigromelunica and D.melanura.
Despite the extensive intraspecific D. micromelanica phylogeny, the same sequence (+) leads to both branches of the group phylogeny. In other words, none
of the existing D. micromelanicu intraspecific phylogeny has been utilized in the
separation of the two derived lines.
Chromosome 2 analysis agrees with the analyses of chromosomes X-left and
3 in demonstrating a phylogenetic sequence from D.micromelanica to D.nigromelanica along one branch, and to the other four species along the other branch,
the order of these four species being the same as indicated by the analysis of
X-left, and agreeing with that indicated by chromosome 3 .
The phylogenetic changes in chromosome 4: The chromosome 4 analysis presents some features which at first sight seem to indicate a distinct difference in
phylogenetic sequence from that found in the other chromosomes (Figure 5 ) .
While two distinct branches arise from D. micromelanica, as in the cases above.
one leading to D. nigromelanica and the other to D. melunura, the phylogenetic
interpretation from that point on is complicated by the fact that BCKLM, the
gene sequence found in D.melanura and in D. paramelunica, does not appear in
D. euronotus, where only BCKLMN is known, despite the fact that the latter
species has been thoroughly studied cytologically. This omission, taken at face
value, would suggest that the D. puramelanica sequence BCKLM probably came
through D. melanura rather than through D. euronotus, and that the BCKLMN
sequence of D. euronotus arose as a side branch from either D. melanura or
D. paramelanicu. An alternate explanation, leading to the same phylogenies as
those deduced on the basis of chromosomes XL and 2, would be to assume that
the missing BCKLM sequence in D. euronotus was originally present in that
species, but was later replaced by the BCKLMN now found. That this alternate
explanation is probably the correct one is indicated by the fact that in the species
334
iD.
ZfJROIIOT[IS ( B C K M ) - C (not known in t h l a s p e c l e a )
!D.
PARAMEUNICA
-
1 D. M E L U I C A
!
I
( l e a s 4R b a s e )
-
(less 4 R baas)
1P
PQ
i
!
!
I
1 1 I 1 I 1 I 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 . 1 1 1
FIGURE
5.-The phylogeny based on inversions and loss in the two arms of chromosome 4 of
the D. melanica group. The general meaning of the symbols is as in Figures 2, 3 and 4. See
tzxt.
D. paramelanica and D.melanica, but in no others, a section of the base of the
right arm of the fourth chromosome is missing. This deletion, comprising approximately ten bands, is clearly s h a m in the hybrid larvae from the cross D. paramelanica females x D.melanura males and its presence or absence can be readily
determined from photomaps or supplementary photographs in the other four
species. Whether this is a true delection, or a transposition to another chromosome
is not clear, but in any event, it indicates that the phylogenetic order for chromosome 4 is D. melanura to D.euronotus to D. paramelanica, thus agreeing with
the order based on the other chromosomes.
The X of D. micromelanica and X-right of the other species: After numerous
attempts to work out the details of the species differences of the X-right arm
through the entire group, the task was found to be so difficult that it was abandoned. It is not certain whether the difficulty encountered was due to the frequently inferior quality of the X chromosome preparations in this group, or because so many X chromosome inversions exist between species (especially among
the first three species in the phylogeny) that they cannot be worked out in detail.
The cytotaxonomic data and conclusions may be summarized as follows: The
data from all critical chromosome analyses (those from chromosome 3 are somewhat ambiguous) indicate the same phylogenetic pattern, with one branch leading from D. micromelanica to D. nigromelanica, and the other from D. micromelanica to D. mehnura and then the other three species in a specific order.
That these two branches are not independent in their origin from D. micromelanica is indicated by three kinds of evidence. First, members of both phylo-
Drosophila melanica GROUP
PHYLOGENY
335
genetic branches share the two-armed, V-shaped X chromosome, and the originally autosomal arm involved in the fusion (“XL”) is the same in both branches.
Second, all five of the derived species in the two branches carry inversion C in
the third chromosome, an inversion not found in D. micromelanica. Third, in
the chromosome 2 phylogeny both branches stem from the same gene sequence
in D. micromelanica (+), although at least ten different sequences now exist
in that species. The same phenomenon exists in chromosomes 3 and 4,but here
it is less convincing as an argument, since only two gene sequences for each of
these chromosomes are known to exist in D. micromelanica.
As mentioned above, the six species can be divided into three groups ( D . micromelanica; D. nigromelanica; other four species) on the basis of their morphology
and reproductive isolation. The above analysis of interspecific inversions not only
sorts out the three subgroups, but also reinforces the grouping. Thus D.micromelanica differs from D. nigromelanica by 18 inversions, and from D. melanura
(the first member of the third subgroup) by 16 inversions, while the species to
species difference among the members of the third group is only five, five and sixplus inversions respectively. The morphological and physiological differences thus
agree very well with the chromosomal differences within the D. melanica group.
The derived phylogeny is particularly interesting when account is taken of the
geographic (northern o r southern) distribution of the species involved. This is
illustrated in Figure 6. It will be noted, that except for the origin of the branch
leading to D. nigromelanica (which species cannot be classified as either northern
or southern), all northern species are derived from southern ones, and southern
species from northern ones. While the reason for this regularity is not known,
it is reasonable to suppose that it involves a utilization of ecologically available
new frontiers. Thus a southern species with a wide distribution could most
readily give rise to an allopatric derivative to the north, since the Atlantic Ocean,
Gulf of Mexico, and to some extent the Great Plains, would tend to block migration in other directions. The derived northern species would in turn tend to be
blocked to the east and west, as was the southern ancestor, and to the north by
climatic extremes. Holwever, following some evolutionary differentiation, the
south would again be open to new speciation, since a new derived species would
be sufficiently different from the original southern ancestor to be able to exist
sympatrically with it.
B O R T H E R N
D. MBLAUURA
D. MICROPIEIANICA
S P E C I E S
D. P A W E L A N I C A
D. WRONORlS
S O U T H E R N
D. HELANICA
S P E C I E S
FIGURE
6.-The relationship between geographical range and origin of species in the D.
malanica group. With the exception of D.nigromelanica, which has a very extensive north-south
range, all species may be classified as northern or southern in their distribution. It will be noted
that, following the phylogenetic scheme based on study of inversions, in every case northern
species are derived from southern ones, and vice versa.
336
H. D. STALKER
In the case of D.micromelanicu, which has already given rise to two species,
there is of course evidence that it may be actively speciating at present, as indicated by the finding of the reproductive isolation between its subspecies. The
presently disjunct distribution of D.micromelanicu would be expected to promote
exactly the type of subspeciation which has in fact been found.
The discussion above does not take into account the possibility of speciation
to the south through Mexico. In fact, both D.micromelanicu and D.melanicu are
found in northern Mexico, but no D.melanicu group species has been recorded
from points further south. Possibly the changing environment has prevented
such southern expansion, or perhaps Neotropical species groups such as the
D.unnulimanu group have indeed been derived from southern D.melanica group
members.
The origin of the D.melanicu group as a whole is unknown. It may have come
from the Palaearctic region, since D.pengi, a member of the group is known from
Japan. Alternatively, the group may have originated in the New World, with
the present distribution of D.p m g i the result of emigration. Studies of D.pengi
and related forms such as the widespread D.virilis and D.repleta groups, the
Neotropical D.annulimana group, the Nearctic D.robusta and D.curbonaria
groups, and the unclassified species D.carsoni may shed more light on the problem. Such studies are in progress.
DISCUSSION
It will be noted from the left half of Table 1 that most of the species in the
group show rather large numbers of intraspecific inversion differences. The discovery of only four such inversions in D.nigromelanica is probably significant,
as this species has been quite thoroughly studied. In the case of D.melanura,
TABLE 1
Distribution of intraspecific inversions among the chromosomes of the six species
of the D. melanica group
No. population samples
structurally heterozygous in various
number of nonhomologous chromosomes
Total inversions
known in various chromosomes
Species
D. micromelanica
D. nigromelanica
D. melanura
D. euronotus
D. paramelanica
D. melanica
Totals
Percentage:
XL-XR
2
3
4
1-2
0-0
1-1
0-1
4-5
1-3
9
3
4
17
1
0
0
0
1
1
0
0
7
0
0
16
0
2
6
18
16
22
19
24
56
70
1
I
4
5
80
100
Total
None
One Two Three Four
14
4
3
13
1
20
3
6 2 0
10
9
56
52
2
6
3
5
10
9
4
35
33
5
5
N
1
7
22
3
26
13
36
1
1
107
100
1
In the left half of the table the figures indicate the total known inversions in each chromosome for each species. In the
right half of the table the figures indicate the numbers of population samples showing no inversion variability, inversions
in one chromosome, inversions in two nonhomologous chromosomes, etc. In the case of D . melanica all population samples
are derived from laboratory stocks, in many instances a single stock representing a population. In the other species almost
all samples are based on studies of various numbers of wild-caught individuals.
Drosophila melanica GROUP
PHYLOGENY
33 7
however, the paucity of inversions undoubtedly reflects the inadequate sampling
of the species. For the group as a whole, 70% of the intraspecific inversions occur
on chromosome 2, and for each of the species except D.paramelanica, that chromosome is the most polymorphic. In the latter species, seven of the X chromosome inversions are associated with the LLSex-Ratio’’
X chromosome, and this
chromosome, with its internally balanced inversions system, has the special advantages of meiotic drive (STALKER
1961). Similar X chromosome complexes are
unknown in the other D. melanica group members.
The concentration of chromosomal polymorphism in chromosome 2 of the D.
melanica group is an example of a phenomenon well known in other Drosophila
species, notably among members of the D. obscura and D.repleta groups. In
many other species. no such pronounced concentration is found; examples of this
latter group are: D. robusta, D. melanogaster, D. willistoni and most of the members of the D. uirilis group.
I n those species with concentration of polymorphism in a particular chromosome, several sorts of explanations might apply.
I. It has been suggested by NOVITSKI
(1961) and BERNSTEIN
and GOLDSCHMIDT
( 1961) that chromosomes already heterozygous for inversions are especially
liable to show breaks (or fail to heal following breakage), near the break-points
of the inversion already present. The latter authors, using X-ray produced rearrangements, have offered evidence to support this view. Whether such structurally heterozygous chromosomes, or chromosome regions, are especially likely
to undergo spontaneous structural changes in nature is not known, since the rate
of production of spontaneous inversions is extremely low, and samples drawn
from nature are probably far from representative because of the selective elimination of most of the new inversions produced.
11. New inversions might be expected to become established in chromosomes
which are already polymorphic because females heterozygous for inversions in
two or more different chromosomes might be selected against because of the production of aneuploid gametes. Such female sterility is known to occur in D.
melanogaster (COOPER,
ZIMMERING
and KRIVSHENKO
1955), D. pseudoobscura
( TERZAGHI
and KNAPP 1960) and D. paramelanica (STALKER,
unpublished).
Female sterility of this sort is however not ubiquitous. In D.robusta, which is an
example of a species in which inversion polymorphism is generally distributed
over all major chromosomes, RILES(1965) has shown that females heterozygous
for two or more unlinked inversions suffer little or no sterility as a result.
Whether other members of the D. melanica group show the female sterility characteristic of D.paramelanica is not known, but its absence in the closely related
D.robusta indicates that they need not.
TERZAGHI
and KNAPP(1960) have made the interesting suggestion that such
female sterility could be an important factor in limiting chromosomal polymorphism to a given chromosome, since in a population in which a certain
chromosome was already polymorphic, inversion heterozygosity in nonhomologous chromosomes would be selected against, while additional inversions in the
338
H. D. STALKER
originally polymorphic chromosome would not. NOVITSKI( 1962) points out,
however, that different populations might build up restrictive systems of polymorphism in different chromosomes, thus leading to species in which there was
a general distribution of polymorphism over all chromosomes.
111. Since inversion heterozygosity tends to reduce recombination and thus
tighten up internally balanced polygenic systems, additional inversions in an
already polymorphic chromosome would tend to further reduce recombination
and breakdown of the balanced polygenic system. On the other hand, new inversions in nonhomologous chromosomes would generally have the opposite effect, by increasing recombination in the first chromosome. Of course at the same
time, such new unlinked inversions could have an adaptive value of their own,
in establishing a tighter balanced system in their own chromosome, but it would
be expected that such reduction in a chromosome in which there was a past history of free recombination, would be less adaptive than strengthening the existing
balanced system in an already polymorphic chromosome.
If either female sterility due to unlinked inversions (I1 above), or breakdown
of establislied polygenic systems (I11above), were of great importance in nature,
then one might expect to find in species in which polymorphism exists on more
than one chromosome that within populations there should be a tendency for one
chromosome or the other to be polymorphic, but not both. The data of WARD
(1952) for D. melanica and STALKER
for the rest of the group are summarized
from this point of view in the right half of Table 1. In this table are shown the
numbers of population samples showing chromosomal polymorphism in no chrodata
mosome, one chromosome, two non homologous chromosomes, etc. WARD’S
for D. melanica are all based on the study of laboratory stocks, some of which had
been maintained in the laboratory for a considerable time before analysis; in
many cases a population is represented by a single stock. STALKER’S
data for the
other five species are almost all based on analysis of numbers of wild-caught flies.
The majority of population samples must be considered inadequate, and thus in
general the chromosomal polymorphism of most populations will be markedly
underestimated. Despite this fact it is significant that 39% of the population samples were polymorphic for two or more nonhomologous chromosomes, and with
more adequate data, this frequency would undoubtedly be greatly increased.
A somewhat different way of examining the available data would be to seek
a correlation of the frequencies of heterozygosis for inversions in two different
chromosomes in the same population. If structural heterozygosis in two different
chromosomes were nonadaptive from a population point of view, it might be expected that the estimated frequencies of heterozygosis in two different chromosomes would show a negative correlation. For three of the species, a total of 30
population samples were suitable for such analysis; 9 in D. nigromelanica, 7 in
D. paramelanica, and 14 in D. euronotus. The data give no evidence of any correlation for D. paramekaniccr and D. euronotus, and in the case of D. nigromelanica, a positive correlation is suggested ( r = +0.6, P = 0.8).
Thus the data in the D. melanica group do not indicate that within populations
there is a strong effective selection against females heterozygous for unlinked in-
Drosophila melanica
339
GROUP PHYLOGENY
versions. While such selection might well be suggested by more data, it seems
improbable that it is a potent force in the restriction of inversion heterozygosity
to a given chromosome.
Explanations I, I1 and I11 above all involve amplification of polymorphism already existing in a given chromosome. If at the time a new species becomes isolated from its ancestor it retains in its populations some of the chromosomal polymorphism already present in that ancestor, then any or perhaps all three of these
explanations might suggest why a series of related species all agree in restricting
most of the chromosomal polymorphism to the same chromosome. However, it is
by no means clear that chromosomal polymorphism is carried over from one
species to the next. Evidence based on species similarities in chromosomal polymorphism seems to indicate that in fact chromosomal polymorphism is not carried over, and that newly isolated species are monomorphic. If a newly isolated
incipient species generally retained some of the polymorphism present in its
ancestor, then one might expect to find cases of two closely related species, both
of which were heterozygous for the same inversion. In the D.melanica group,
with its large numher of intraspecific inversions, there is not a single case of two
different species being heterozygous for the same inversion. I n the D. repleta
group, with its 46 analyzed species, WASSERMAN
(1960) found only three such
cases, and only one has been found in the nine species of the D. uirilis group
(STONE,GUESTand WILSON
1960). Even these four examples may be the result
of secondary hybridization of incompletely isolated species, rather than polymorphic origin of species.
Of course, as WASSERMAN
(1960) has pointed out, the general absence of such
cases of shared inversion heterozygosity does not in itself prove that new species
are monomorphic, since a new species retaining some of the polymorphism of its
ancestor might, following further incorporation of new inversions, build up a
unique chromosomal polymorphism of its own, and by the loss of one of the
ancestral sequences, fail to give evidence of its polymorphic origin. Thus, if the
ancestral species were A/+ (carried both inversion A and the Standard sequtmce), the derived species might at the time of its isolation also be A/+, but
become
by later incorporation of new inversions and loss of the ancestral
progressively A/AB, AC/ABD, etc. It would now be homozygous for inversion
A, but would never have passed through a monomorphic stage. Although such
a series of events could certainly occur, it seems improbable that they represent
the usual situation, and could thus explain the general failure to find related
species heterozygous for the same inversion.
If it is assumed that in most instances new species are structurally monomorphic, then the tendency of related species to show restriction of their polymorphism to a specific chromosome (e.g. chromosome 2 in the D. melanica
group), seems to require either that the origin of new inversions is concentrated
in a specific chromosome, and this nonrandom origin persists through many
species. or else that the origin of new inversions is random, hut that certain
chromosomes, because of their past history of reduced recombination through inversion heterozygosity in ancestral species, carry a complex of polygenes which
+,
340
H. D. STALKER
are especially likely to form adaptive complexes when tied up by inversions;
with the result that the randomly occurring new inversions are incorporated only
if they occur in a specific “favorable” chromosome.
In summary, neither the restrictive pattern of inversions within species, nor
the similar restrictive patterns among related species can be easily explained. The
general failure to find shared polymorphism in related species suggests new
species are generally monomorphic at their inception, but does not prove it, and
if such initial monomorphism is the general rule, then the similarity in the restrictive patterns between related species requires special explanation.
It is worth pointing out that in the D.melanica group most of the presently
existing intraspecific polymorphism must be of relatively recent origin, that is,
has developed since the differentiation and isolation of the species involved. The
reasoning behind this conclusion rests on two facts. First, there is little or no involvement of existing intraspecific phylogenies in those chromosome lines leading from one species through another and on to a third, or in the case of D.
micromelanica, leading from one species to two others. This is a highly improbable situation if in fact the present-day intraspecific inversion polymorphism had
existed at the time of initial speciation. Second, with few exceptions (such as
BCKLM in the fourth chromosome of D.euronotus), there are no missing or
“hypothetical” sequences in the intraspecific phylogenies, again strongly suggesting their recent origin.
LEVITAN(1962, 1964) has recently found a structurally homozygous stock of
D. robusta which carries a maternally transmitted, apparently cytoplasmic factor, capable of inducing large numbers of new chromosomal aberrations (1,000
in less than three pears) of all sorts, including nearly 40% inversions. This interesting discovery raises the question of whether or not some of the extensive
intraspecific phylogenies in the D.melanica group, which are presumed to have
arisen relatively recently, could have originated in this way. It is the opinion of
the author that this is an unlikely explanation, since in a species or population
rapidly acquiring new inversions, there would be little opportunity for the adaptive adjustments necessary f o r their incorporation, and one would expect that repeated re-inversion in the same chromosome would ultimately result in intraspecific phylogenies with many missing steps due to loss of intermediate chromosome types. The D. melanica group does not show such frequent gaps in its intraspecific phylogenies.
It is a pleasure to acknowledge the technical assistance of MR. DUANETUPY
and MISSJ E A N
COUGHLIN
in the preparation of chromosome smears, and the kindness of DR. WILLIAMB. HEED
in supplying unpublished data on inversion frequencies in the D.curdini group. The author is
who should however
a h grateful for a critical reading of the manuscript by DR. H. L. CARSON,
not be held accountable for the ideas expressed.
SUMMARY
The phylogenetic relationships of the six Nearctic members of the D.melanica
group are analyzed by the use of salivary gland chromosome similarities in banding pattern. The chromosome analysis leads to the division of the group into
Drosophila melanica GROUP
PHYLOGENY
341
three sub-groups, containing D. micromelanica in one, D. nigromelanica in a
second, and D. melanura, D. euronotus, D. paramelanica and D. melanica in a
third. The same three sub-groups might be deduced on the basis of morphological
and physiological characteristics.
Because of its close cytological relationship with members of other species
groups, and because it is the only D. melanica group member with a singleelement X chromosome, D.micromelanica is considered to be ancestral for its
group. The group phylogeny has two branches, one leading from D. micromlanica to D. nigromelanica, the other from D. micromelanica to D. melanura
to D. euronotus to D. paramelanica to D. melanica. Except for D. nigromelanica,
which cannot be classified as distinctly northern or southern in its distribution,
all northern species are derived from southern species, and southern species from
northern species.
The six members of the group reveal a total of 141 inversions of which 80 are
heterozygous within species, and 61 (partially estimated) constitute permanent
homozygous differences between the species. The average number of inversions
of all kinds per species, 23.5 is considerably higher than that found in the D.
repleta, D. uirilis and D. cardini groups (3.1, 13.0 and 9.6 respectively).
In the D.melanica group the inversions are very unevenly distributed over the
chromosomes, with chromosome 2 showing the most variability between species,
and in all but one case, within species. The possible reasons for this inversion
concentration are discussed.
Despite the fact that very extensive and complete intraspecific inversion phylogenies exist, these intraspecific phylogenies are for the most part by-passed by
the phylogenetic lines leading from one species to the next, suggesting that the
six species may have become differentiated relatively rapidly, with the intraspecific inversion polymorphism of recent origin.
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