Joological Journal of.the Linnean Society (1984),80: 4 2 1 4 3 5 . With 6 figures
Evolutionary relationships within three
Tilapiine genera (Pisces: Cichlidae)
B. J. McANDREW AND K. C. MAJUMDAR
Institute o f Aquaculture, University OJ' Stirling, Stirling FK9 4 L A , Scotland
Recezued March 1983, accepted Jar Publzcation August 1983
Species of thc cichlid genera Tilapia, Sarotherodon and Oreochromis were studied by electrophoresis at
25 enzyme loci. T h e levels of similarities within and between genera, and the distribution of genetic
identities at individual loci are similar to those found in other fish species. T h e evolutionary
relationships between thc species remains equivocal, and it is still not possible to decide between the
hypotheses of Trewavas (1980) or that of Peters & Berns (1978, 1982). Further work needs to be
undertaken on additional Tilapia and Sarotherodon species. The unexpected relationship of 0.jipe to
the other Oreochromis species is probably the strongest argument against Trewavas' hypothesis. But
the close similarity of the maternal mouth brooders to each other, and the closer relationship of S.
galilaeus to the Oreochromis species than to the Tzlapza does not favour the hypothesis of Peters &
Berns (1982). T h e difficulties in obtaining pure species from the wild due to widespread
introduction of non-endemic species are likely to hinder progress in evolutionary and aquacultural
studies.
KEY WORDS:--Evolution
Oreochromis
~
Sarotherodon
-
electrophoresis
Tzlapia.
-
-
dendograms
-
genetic distance
enzymes
-
CONTENTS
Introduction .
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Materials and methods.
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Results
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Genetic identity ( I n ) and genetic distance (Dn) .
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Unweighted pair group method and arithmetic means (UPGMA)
. . . . . . . .
Fitch & Margoliash method .
. .
Wagner trees of Farris trees using frequency coded data
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Conventional Wagner procedure using binary coded data .
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Discussion.
Acknowledgements
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42 I
423
425
425
425
427
427
428
429
433
434
INTRODUCTION
In recent years one group of fish within the Cichlidae has been receiving
much renewed attention. The tilapias are a group of species which have
immense potential in tropical aquaculture, and from their natural range within
Africa they have been transplanted so that one species or another is to be found
in most tropical and subtropical countries of the world. Despite the wide
environmental tolerances of various species, the 70 species show relatively little
+
0024-4082/84/04042 1 + I5 $03 00/0
4'2 I
01984 The Linnean
Soctet\ of London
422
B. J. McANDREW AND K. C . MAJUMDAR
morphological diversity and this has led to much confusion in species
identification (McAndrew & Majumdar, 1983) and their phylogenetic
rela tionships.
The whole group was initially placed in the genus Tilapia but since the 1920s
there have been numerous attempts by taxonomists to divide them into a
number of smaller groups. (Regan, 1920; Thys van den Audenaerde 1968, 1971;
Trewavas, 1973, 1980, 1982; Peters & Berns, 1978, 1982). Initially these
divisions were based on structural characteristics such as the number of gill
rakers, jaw structure and the teeth on the pharyngeal bone. More recently as
information about the fishes’ biology has become known, subdivisions have been
based on differences in breeding and brooding behaviour. The most recent
classification by Trewavas (1982) divides the tilapias into three genera: Tilapia
which consists of substrate spawners, Sarotherodon which includes paternal and
biparental mouthbrooders and Oreochromis which is restricted to maternal
mouthbrooders, the latter genus being divided into four subgenera. This
classification follows that given by Thys van den Audenaerde (1968) which gave
a different rank to the groups, leaving Tilapia as the genus and Sarotherodon and
Oreochromis as subgenera. These ideas clash with those of Peters & Berns (1978,
1982) who believe that any of these subdivisions are not justified and that the
various forms should all be called Tilapia based on their ideas of the evolution of
the group.
It is generally agreed that mouthbrooders evolved from a substrate-spawning
ancestor. Peters & Berns (1978, 1982) (Fig. 1A) suggest that substrate spawners
have given off mouthbrooding branches from time to time, and it is the time
since this branch occurred which determines the state of advancement of
mouthbrooding characteristics. The most ancient branch
(‘older’
mouthbrooders) are represented by the maternal mouthbrooders, compared to
the more primitive breeding characteristics of the more recently branched
‘younger’ mouthbrooders. Trewavas ( 1980) puts forward an alternative
hypothesis (Fig. 1B) in which a Tilajia-like ancestor gave rise to a
mouthbrooding branch which itself quickly divided into two: one which
remained conservative regarding breeding behaviour but not other
characteristics (Sarotherodon) and a more progressive branch which led to the
Oreochromis.
I n recent years data on molecular variation have proved very useful in
tackling systematic problems and it is generally found that phylogenetic trees
derived from molecular data are similar to those derived from anatomical and
morphological characteristics (Mickevich & Johnson, 1976). A number of
workers have used molecular data on the tilapiines as an aid to species
identification (Chen & Tsuyuki, 1970; Avtalion, Pruginin & Rothbard, 1975;
Avtalion et al., 1976; McAndrew & Majumdar, 1983). The only previous study
which has compared evolutionary relationships in this group was by Kornfield et
al. (1979) which confirmed the broad taxonomic relationships derived from
morphological data, although only utilizing one species from each of the three
genera.
In this paper we will consider the evolutionary trees produced by three
different dendrogram construction methods, using 25 enzyme loci in nine
different tilapiine species. At least one species from each of the three genera
proposed by Trewavas (1982) are included. The results are discussed with
427
TILAPIINE EVOLUTIONARY RELATIONSHIPS
Tflapfa
Substrate
spawners
Genus
Mput h brooders
‘Younger
Older’
Tilapa
Sarotherodon Oreochromis
Substrate
spawners
Paternal and
biparental
Maternal
mouthbrooders
Figure IA, B. Diagrams to show the differences between the hypotheses of Peters & Berns I 1978,
1982) and ‘Irrwavas (1980). ‘The diagrams have been modified from those of Trewavas (1980’ to
include the latest generic status of the different mouthbrooders (Trewavas 1982).
reference to similar studies on other groups of organisms. Attention is paid to the
way these results compare with existing ideas about evolution within this group.
MATERIALS AND M E T H O D S
T h e preparative procedures and electrophoretic techniques are given by
McAndrew & Majumdar (1983). Table 1 gives the species, their sources a n d
denotes populations which were sympatric. T h e allele frequency d a t a a n d relative
mobilities of the loci studied have already been published by McAndrew &
Majumdar ( 1983).
Genetic identity and distance was calculated from the allele frequencies using
the formulae given by Nei (1972). Both the genetic identity (In) and genetic
are given in Table 2. Standard errors for Dn are also given (Nei,
distance (Dn)
Table 1 . T h e table shows the species studied a n d their sources and denotes
populations which were sympatric. Species with (Bamburi) after their name
came from the collection a t Baobab Farm, Bamburi (see Acknowledgements)
-~
Speciri
~~~~
Source
~~~~~
‘Tilapia zillii
.Caratherodon galilaeus
Oreoihromi~Subgcnus Oreoihromis
andersonii
Oreochromis (O.,
Oreoihromis ( 0 . aureus
Oreochromis ( 0 .j j i p e
Oreochromis 0. mossambicus
Oreoihromis O., niloltcuJ
Oreochromis ( 0 ., spilurus
OreochromiJ Xjasalapia
Oreochromis (J+’. , macrochir
$8
Egypt (Lake Manzala)
Kenya (Lake Turkana) (Bamburi
Botswdna Okavango) (Bamburi
Egypt (Lake Manzala)
Kenya (Lake Jipe) (Barnbun)
Aquanst stock
Egypt (Lake Manzala)
Kenya ( R T a n a ) (Bamburi)
Botswana [Okavango) (Bamburi
*
t
t
*
*
B. J. McANDREW AND K. C. MAJUMDAR
424
Table 2. Genetic distances (on)
and their standard errors (below diagonal);
genetic identity (In) (above diagonal) for nine tilapiine species analysed for 25
enzyme loci
0 . ander.
0 . andersonii
0. aureur
S.galilaeus
0 .jipe
0. macrochir
0 . mossambicus
0. niloticur
0. spilurur
T. zillii
0 . aur.
-
0.297
f0.119
0.429
f0.149
0.931
+0.253
0.350
50.132
0.171
f0.084
0.262
k0.108
0.311
k0.123
0.641
+0.194
0.732
-
0.304
fO.121
0.651
f0.194
0.249
f0.107
0.263
fO.111
0.115
f0.068
0.152
- 0.080
0.542
+0.173
+
S . galil.
0.651
0.738
~
0.671
0 .jipe
0 . macro. 0 . moss.
0 . nil.
0 . spil.
7.zillii
0.394
0.521
0.705
0.780
0.873
0.769
0.768
0.891
0.732
0.859
0.526
0.582
0.511
0.768
0.687
0.806
0.768
0.532
0.517
0.429
0.564
0.517
0.391
0.768
0.843
0.897
0.566
-
0.799
0.784
0.486
-
0.925
0.668
-
0.647
0.436
k0.151
-
~
f0.199
0.263
0.659
~
kO.111 f0.196
0.375
f0.137
0.216
f0.098
0.264
+0.111
0.631
+0.192
0.846
0.262
+0.234
f0.109
0.572
f0.178
0.660
k0.197
0.938
+0.254
0.171
0.224
0.085 f 0.100
0.108
0.243
k0.066 f0.103
0.569
0.721
f0.179 fO.210
0.077
50.052
0.403
f0.142
1971). Three different methods were used to produce dendrograms from the
electrophoretic results. This was because there is still some doubt as to the best
procedure for this type of data (Prager & Wilson, 1978; Felsenstein, 1982). The
three different methods were the unweighted pair group arithmetic average
clustering method (UPGMA), the phylogenetic tree construction method of
Fitch & Margoliash (1967) (F-M) and the conventional Wagner procedure after
Farris (1970).
The UPGMA utilized the distance matrix in Table 2 and was calculated
using the Clustan B computer package. The UPGMA method does not indicate
the amount of change along individual lineages and assumes homogeneity of
rates of evolution among the lineages compared.
The F-M procedure was calculated with a programmable calculator. The
choice of the best tree from those produced was the one with the least difference
from the input data. The tree chosen was that with the smallest F value (Prager
& Wilson, 1978). F is defined as F= lOOf/I where f is the sum of the absolute
value of the differences between output and input values and I is the sum of the
input values.
The Wagner procedure of Farris (1970) utilized the original allele frequency
data (McAndrew & Majumdar, 1983) both binary coded (if allele present
equals 1, if absent equals 0) as well as frequency coding to one decimal place.
The allele frequency data were used in preference to the distance matrix because
it was felt that more systematic information would be retained. The Wagner
algorithm works on the principle that the best estimate of a monophyletic group
is the tree with the smallest number of character transformations, and like the FM procedure does not assume homogeneous rates of evolution in the lineages
concerned. The Wagner trees were calculated using the Wagner procedure
p r o g r a m (WAGPROC) based on Swofford ( 1981) . This program produces
an undirected Wagner network which is rooted at the mid point of the greatest
patristic distance, and of the minimum evolutionary length.
TILAPIINE EVOLUTIONARY RELATIONSHIPS
425
60-
50-
0
0:2
0.'4
1.0
Genetic identity
Figure 2. Histogram showing the percentage of loci within a given range of genctir identity among
nine species from three tilapiine genera (Trewavas, 1982).
RESUL'I'S
Genetic identity ( I n ) and genetic distance ( D n )
The calculated values of In and Dn with standard errors are given in Table 2.
The values for Dn will be used in discussion where possible as these allow an
estimate of the level of error. The most similar pair are 0. niloticus and 0. spilurus
(Dn=0.077f0.056) and the least similar are 0 . j i p e and 7.zillii
( D n= 0.938 f0.254) although the values overlap between all other 0.jipe
comparisons. In general, the Dn increased with increasing rank. The mean Dn
within the Oreochromis was Dn=0.369 including 0 . j i p e but only Dn=0.217 if
0.j;Pe is removed. Average intergeneric differences in Dn between Oreochromis
and Sarotherodon were Dn = 0.360, between Oreochromis and Tilapia it was
Dn = 0.607. The difference between Sarotherodon and Tilapia was Dn = 0.63 1. The
average Dn between the three genera was Dn = 0.533.
The distribution of single locus In can be seen in Fig. 2 and is the typical Ushaped pattern which has been seen by many other such surveys (Avise &
Smith, 1977). The majority of the loci studied (89.5'1;) have either identical
allelic composition or are else completely distinct with unique alleles.
I;nu,eighted pair group method with arithmetic means (C'PGMA)
The product of the UPGMA agglomerative clustering procedure (Fig. 3 ) is
based on the Dn measures in Table 2, and the dendrogram produced shows the
majority of the mouthbrooders clumped together. However, the position of
0. jipe is totally unexpected being more remote from the other mouthbrooders
than even 7. zillii. Similarly, the position of S. ga1ilaeu.r amongst the
B. J. McANDREW AND K. C. MAJUMDAR
426
3
Nei:
Gormaneta/:
I
I
15
2
I
I
I
I Years x lo6
I
0m e
TI/////
S go/i/aeus
0 macrochir
0 spilurus
0 nl/ot/cus
0 oureus
0 rnossambicus
0 andersonii
I
0.0
I
0.6
I
0.4
I
0.2
I
0
Genetic distance
Figure 3. Phenogram showing relationships of nine species from three tilapiine genera (Trewavas,
1982) according to UPGMA method of cluster analysis using species distance matrix Table 2, with
two different estimates of evolutionary time.
mouthbrooders is unexpected if the Sarotherodon are held to be ancestral to the
maternal mouth-brooding species. The scale on this Fig. 3 is in Dn and if one
assumes a reasonably homogeneous evolutionary rate within the species, it may
also be used as an evolutionary time scale. There are two different estimates as to
the time scale which is most appropriate; a Dn value of 1 is equivalent to 5 x lo6
years (Nei, 1975) whereas other workers believe it to approximate to 18 x lo6
years (Gorman, Kim & Rubinoff, 1976). Both scales have been included in the
figure but the latter seems to have proved to be close to the known geological
L
0.5
I
I
0.4
0.3
I
0.2
I
0.1
I
0
Average genetic distance
Figure 4. Phenogram showing relationships of nine species from three tilapiine genera (Trewavas,
1982) according to the Fitch-Margoliash phylogenetic tree construction procedure using species
distance matrix (Table 2). Each apex point is placed a t a n ordinate value representing the average
of the sums of genetic distances in the lines from that apex.
'I'ILAPIINE EVOLUTIONARY RELATIONSHIPS
42 7
events in previous studies (Soule & Gorman, 1974; Gorman et al., 1976; Vawter,
Rosenblatt & Gorman, 1980).
Fitch & Margolaash method
This tree (Fig. 4) was selected from 20 others as the one with the lowest Fvalue. In general the tree is very similar to the UPGMA and Wagner trees
except that S. galilaeus is ancestral to the majority of the mouthbrooders.
Olaochromis jipe is still very different to the other mouthbrooders and is further
away than T. Zillii.
Wugner trees or Farris trees using frequency coded data
The two trees of the same minimum length were (Fig. 5A,B) produced by
using the allele frequency data and both have the same minimum length and
0 andersonii
a
0 mossarnbicus
S galilaeus
0 rnacrochir
0 spilurus
0 aureas
L
0niloflcus
0 andersonii
0 rnossamhicus
0macrochir
0 spilurus
m,
0 aureus
0niloficus
T ziJli/
CL
13.14
L
8.27
12.41
16.55
O/lm
Distance from root
Figure 5, A, B. Phenograrns showing relationship of nine species from three tilapiine gencra
(Trewavas, 1982 according to Wagner tree procedure of Farris (19701. These trees have been
constructed using the allelr frequency data from McAndrew & Majumdar ' 1983 .
B. J. McANDREW AND K. C. MAJUMDAR
428
Q ondersonii
0.mossombicus
Ad-
0 mocrochir
0 spilurus
0 oureus
0 nifoticus
I
0.
Iipe
0.ondersonii
B
0 mossombicus
0 mocrochir
0 spilurus
0 oureus
rl
C
S golifoeus
I
0.nifoticus
’
-
,
0 ondersonii
0.mossombicus
0 mocrochir
0 spifurus
S.gofifaeus
ri
0 oureus
0 niloficus
T zifhi
0 jipe
Distance from root
Figure 6A, B, C. Phenograms showing relationships of nine species from three tilapiine genera
(Trewavas, 1982) according to the Wagner tree procedure of Farris (1970). These trees have been
constructed using the allele frequency data from McAndrew & Majumdar (1983) coded in a binary
fashion.
goodness of fit. The trees produced differ in the relative positions of S. galilaeus
to the 0. spilurus, 0. macrochir branch.
Conuentional Wagner procedure using binary coded data
In all, six trees of the same minimum length and goodness of fit were
produced. Only three have been reproduced, Figs 6A, B & C since the other
‘I‘ILAPIINE EVOLUTIONARY RELATIONSHIPS
129
trees showed only minor positional changes of some of the arms. One tree had a
branching pattern and arm lengths for all species identical to the tree in Fig. 6A
except that the positions of 0. aureus and 0. niloticus were reversed. The tree in
Fig. 6B had a variant in which the 0. aureus and S. galilaeus branches changed
position with 0. niloticus. The final tree had the positions of 0. aureus and 0.
niloticus reversed in comparison with the tree shown in Fig. 6C.
In general the two Wagner methods give similar results in that it is the
position of S. galilaeus within the mouthbrooders which is being varied. Only
one tree, Fig. 6A, shows S. galilaeus ancestral to the maternal mouthbrooders
other than 0.jipe. I n the frequency analysis 0. aureus and 0 . niloticus are always
ancestral to S. galilaeus and it is the position of the 0. macrochir, 0. spilurus branch
that changes position with S. galilaeus. I n the binary coded data (apart from the
tree in Fig. 6A) S. galilaeus is somewhere close to 0. aureus and 0 . niloticus and
always ancestral to the 0. macrochir, 0. spilurus pair.
DISCUSSION
It is generally considered that phylogenetic trees based on morphological and
allozyme characters produce similar results, despite the fact that the underlying
genetic bases for the two sets of characters are likely to be quite different. T h e
relative ease and reliability of electrophoresis for assessing the levels of intraspecific and inter-specific variation has meant that i t has been used widely in
such studies. However, the technique does have a number of limitations.
Electrophoresis will always underestimate differences since only a percentage
(27%) of the codon changes will actually produce detectable differences (King
& Wilson, 1975). The number of loci used in systematic studies is also very
important. The U-shaped distribution of the percentage of loci with a given In
in Fig. 2 clearly shows that the majority of the loci are in fact identical or
unique for the different species.
The main implication of this is that the validity of the systematic information
lies in the number of loci studied. More information will be available from
increasing the number of loci rather than the number of individuals within a
species.
Similarly increasing the number of loci will also decrease the size of the
standard error associated with calculating Nei’s coefficients and lead to a more
accurate interpretation (Nei, 1971; Thorpe, 1982). T h e U-shaped distribution is
very similar to others produced from studies on other groups of sibling species,
species, and genera, and appears to be a common pattern for most outcrossing
sexual organisms.
The levels of In and D n both within and between genera are similar to other
studies of fishes (Kirpichnikov, 1981: 237). The only other evolutionary study
on tilapiines was by Kornfield et al. (1979) which studied a number of cichlids
endemic to the Sea of Galilee. Kornfield et al. ( 1 9 7 9 ) obtained In=O.92 between
0. aureus and S. galilaeus, In = 0.48 between T. zillii and S. galilaeus and In = 0.49
between T. Zillii and 0 . aureus. These compare with In=O.74, In=O.52 and
In = 0.58 respectively in this study. These two independently derived sets of
similarities are interesting in themselves since they underline the variation
inherent in using electrophoresis as a systematic tool. The differences are
probably a result of the different loci compared in the two studies. Sarich (1977)
430
B. J . McANDREW AND K. C. MAJUMDAR
noted that the choice of loci can dramatically affect the estimate of similarity.
Comparisons between loci common to both studies will be informative and
may show the level of divergence which has occurred since these populations of
tilapia have become reproductively isolated. Of the 12 loci which can be
identified as common between the two sets of data, five show some difference
between one or all of the species involved. Sarotherodon galilaeus is polymorphic at
the Pgi-1 & 2 loci but is fixed for the more common allele at both Pgi loci and a
faster 6 Pgdh allele in the study of Kornfield et al. (1979). Tiliapia eillii is fixed
for a slower Ak allele and appears to be fixed for a different Est-2 allele in
Kornfield’s study. Finally 0. aureus is fixed for the more common of the two Ak
alleles which Kornfield et al. (1979) found in their study. A number of
differences occur at the Est-2 loci; it appears that the Israeli population of
S. galilaeus and 0. aureus have more alleles a t this loci but no direct comparisons
can be drawn as no standard exists between the two sets of data.
If an Zn value is calculated for each of the three species based on the common
loci, values of Zn=O.99, 0.96 and 0.92 are found for 0. aureus, S. galilaeus and
7. zillii respectively.
These values are interesting and are similar to the differences which have
been found between populations in other fish species studied (Kirpichnikov,
1981). However, these results must be treated with some caution because of the
inherent errors resulting from the small number of loci and possible differences
in the electrophoretic techniques in the two laboratories. The results do show
that some divergence has occurred since these populations were isolated when
changes in the River Nile drainage caused Lake Turkana and the northern
extension of the rift valley into Israel and Syria to be left with no direct contact
with the main river.
The construction of dendrograms from electrophoretic and sequencing data
has been receiving increasing attention. However, there is still some doubt as to
the best method of analysing such data. Prager & Wilson (1978) have shown
that there are distortions caused by the different methods and suggested that the
Fitch-Margoliash method was the best choice to use with electrophoretic
distance data. The UPGMA method was said to be on average better than the
Farris method but not quite as good as the F-M method. The different methods
also make different assumptions about the homogeneity of evolutionary rates
within individual lines. The F-M and Farris methods, although different
procedures, make no assumption about the rate of evolution within any line,
whereas the UPGMA method produces a phenogram which can only be
assumed to represent an evolutionary tree if the rate of evolution for the
different proteins is constant in all lineages. More recently Felsenstein (1982)
suggested that the different methods for inferring phylogenies should be
evaluated using statistical criteria, as this might give a clearer impression of how
accurately these methods estimated the phylogeny. The various methods make
different sets of assumptions concerning the characters being used. Felsenstein
( 1982) thought that a standard statistical approach such as maximum likelihood
would produce a method, particularly for electrophoretic data, whose properties
are known and which would give some degree of statistical confidence. Despite
this, little work has been done to develop statistical models which resect the
complexity of the evolutionary processes.
Because of the doubts on their relevance, all three of the commonly-used
TILAPI INE EVOLUTIONARY RELATIONSHIPS
43 1
dendrogram construction techniques have been used in this study, and in
general they show very similar patterns of evolution. Some species, 0.j;Pe and
T. zillii and the 0. andersonii and 0. mossambicus pairing maintain their relative
position no matter which method is used. T h e majority of the differences
between the various methods occur with the relative grouping within Oreochromis
and the relationship of S. galilaeus to this genera.
It is generally agreed that mouthbrooding species have evolved from substrate
spawning ancestors. Sarotherodon species also exhibit characteristics intermediate
between substrate spawners and maternal mouthbrooders, particularly in their
reproductive behaviour and in the regression of substrate-spawning
characteristics in their larvae such as the adhesive layer on the eggs and
adhesive glands on the larval head. (Peters, 1965; Peters & Berns, 1978, 1982).
However Peters & Berns (1982) believe that mouthbrooding has occurred a
number of times from substrate spawners possibly from different ancestors and
at different times. These authors demonstrate that in breeding characters
today’s Sarotherodons are closer to the Tilapia than the Oreochromis and propose
that Sarotherodons have only recently split from the Tilapia ancestor. Trewavas
(1980) put forward an alternative theory in which Tilapia gave rise to a
mouthbrooding branch which soon divided into two: a more conservative
branch, leading to the ,Sarotherodon, and a progressive one leading to the
Oreochromis.
I t should be possible to differentiate between the two hypotheses. From Peters
& Berns (1978) explanation one might expect S. galilaeus to be closer to the
substrate spawning T. zillii, although this may not be its direct ancestor. Peters
& Berns (1978) hypothesis does not anticipate that all maternal mouthbrooders
will be closely related as they may have evolved from different ancestors or even
from the same ancestor at different times. This should be evident in the
electrophoretic analysis because once reproductive isolation has taken place,
mutations accumulate at a regular if stochastic rate. Selection may act on
some of these new mutations but is unlikely to cause any major distortions when
a large number of loci are studied. The Peters & Berns hypothesis would predict
larger differences between the mouthbrooders than would be expected by a
monophyletic model of evolution. The lack of availability of more species of
Tzlapia and Sarotherodon does not make comparisons within the genera possible.
However, it appears that S. galilaeus is much closer to the majority of Oreochromis
species than it is to 7.. zillii. This might suggest that Peters & Berns [ 1978)
hypothesis is n o t correct in this case.
The position of 0 . j i p e in all the various trees is unexpected as it is known to
be a maternal mouthbrooder. The fact that 0 . j i p e is so different might suggest
that this species has evolved from a different ancestral substrate spawner, and
may be a candidate for one of Peters’ ‘Older mouthbrooders’. O . j $ e was first
described by Lowe (1955) as one of three new species (and one subspecies) of
the 0. mossambicus complex from Lake Jipe and the Upper Pangani River.
Oreochromisjipe was morphologically similar to one of the other species 0. girigan
but had quite different pharyngeal teeth, more similar to S. galilaeus than to the
‘mossambicus’ complex in which these other species were thought to belong.
Trewavas (pers. comm. 1983) considers 0. hunteri, 0.j p e , 0. girigan, and
0. pangani to be a little group apart from the rest of the Oreochromis in that they
have a higher vertebral number (31-34, mode 33) as compared with the rest of
43 2
B. J. McANDREW AND K. C. MAJUMDAR
the genera (27-32, mode 30), and a characteristic body coloration.
The extreme position of 0. jipe would not be expected from Trewavas’ ( 1980)
hypothesis and could be explained by this species having become isolated at an
early stage from the other mouthbrooders, with local conditions causing the
accumulation of a large number of unique alleles within this species, but there
are no apparent reasons to suspect that this might be the case.
Hybridization is well known amongst the tilapiines. The differences in 0.jipe
could well be the result of a combination of new alleles from a number of
species. The majority of the differences between 0.jipe and the other Oreochromis
are a t uniquely fixed enzyme loci. The heterozygosity of 0. jipe is similarly only
the second highest within the species studied. This would not suggest a recent
hybridization event with anything but a very closely related species.
A number of authors have noted that hybrid populations often contain alleles
not present in either of the parental populations (Hunt & Selander, 1973;
Sage & Selander, 1979). Two mechanisms have been proposed to explain the
presence of these new alleles. The first is that there might be an increased
mutation rate in the hybrid (Thompson & Woodruff, 1978) or that they might
result from intragenic recombination between different alleles of the parental
populations (Watt, 1972; Morgan & Strobeck, 1979; Tsuno, 1981). Recently
Golding & Strobeck (1983) have produced a model which shows that intragenic
recombination at heterozygous loci can generate new unique alleles in such
hybrid populations. However, this is likely to increase the level of heterozygosity
still further in the population.
It is possible that any increase in heterozygosity has been subsequently
reduced because of the loss of alleles due to selection and or allelic repression
(see Whitt, Philipp & Childers, 1973, 1977).
There is little evidence to show that 0.j;Pe is of hybrid origin. The fact that
0 . j i p e is polymorphic for anal spine number (3 or 4) (Lowe, 1955) is unusual
amongst the tilapias; the only other species which show this are 0. hunteri from
Lake Chala and 0. korogwe from the Pangani river. However, this is in no way
confirmatory that any of these species are of a hybrid origin. It is obvious that a
much more detailed study of 0 . j i p e and its closely related species is needed to
determine whether they are a discrete group apart from the other Oreochromis or
whether O.jt$e is an unusual case caused by some hybridization event with an
unknown species. These facts will be needed before any firm conclusions about
the relationship of 0.jipe and its close relatives with the other Oreochromis are
formed.
The overall picture from the various trees must be treated with some caution,
because the trees produced cannot give an exact picture since many species are
missing from the analysis. It is obvious that some species essential for critical
consideration are missing. These omissions will lead to differences between the
trees particularly when each analysis used the data in a slightly different way.
The small number of species considered will affect the results for the
‘mossambicus’ complex because only two close members of this group are present,
0. mossambicus and 0. spilurus, and a more distant relative, 0. andersonii. This
complex is endemic to the rivers of the East African coast and comprises a
number of localized populations from 0. spilurus (Kenya) in the north to
0. mossambicus (South Africa) in the south. It is thought that the fish have
moved along the coast from estuary to estuary especially during times of flood.
TILAPIINE EVOLUTIONARY RELATIONSHIPS
$33
The apparent differences between 0. mossambicus and 0. spilurus could well be
caused by the fact that they are at opposite ends of a range. The differences could
also be Iarger than is actually the case in the wild because the 0. mossambicus
stock was very inbred, coming from aquarist stock, and was monomorphic for
most loci. This could have decreased similarities because any polymorphic loci
may have had shared alleles. Similarly, the alleles fixed in this population may
not all be the common wild allele, because sampling error and bottlenecking of
the populations in captivity may have increased the frequency of less common
alleles so that some of them became fixed.
The geological history of this group of fish is little known. Trewavas (1937)
described a fossil Tilapiine with four anal spines from a Pleistocene deposit (2.5
million years old) at Rawe near Lake Victoria and concluded that it belonged
to 0. spilurus niger. Greenwood (1951) discussed a fossil tilapiine with four anal
spines from Miocene deposits (7-26 million years ago) and considered they were
most closely related to 0. mossambicus. This tends to suggest that relatively little
morphological change has occurred since these times. Figure 3 shows the
UPGMA dendrogram and two estimates of evolutionary time. The estimate of
Gorman et al. 1’1976) (Dn= 1 equivalent to 18 million years) seems to be most
appropriate as it appears to be backed by some geological evidence (Yang et al.,
1974; Gorman et al., 1976; l’awter et al., 1980). Using this time scale it would appear
that the split between Tilapia and the mouthbrooders occurred some 10 million
years ago and that the main speciation within the maternal mouthbrooders
occurred around 5 million years ago. These estimates seem conservative when
the fossil record already shows a mouthbrooding species as long ago as 7 million
years. Indeed the use of such estimates of evolutionary time are fraught with
many errors. Temporal divergence estimates assume that one is dealing with a
constant linear function. Corruccini et al. (1980) have demonstrated that the
relationship between Dn and time is distinctly non-linear. The errors involved
in this type of comparison are probably too great to draw any meaningful
conclusions, particularly as there are no known geological events to act as an
independent estimate of evolutionary time.
It is obvious that many more species need to be analysed before a reliable
evolutionary tree within this group can be developed. However, the ability to
collect pure samples of any tilapiine species is becoming more and more
difficult as uncontrolled introductions of new species and man-made
disturbances lead to a widespread introgression of new genes into pure tilapiine
stocks. This has obvious consequences for taxonomic and evolutionary studies,
in that it will become progressively more difficult to separate endemic pure
species from hybrid populations. I t also has a profound long-term effect on man
because unique gene pools are being lost before anyone can identify their
potential for exploitation directly or as a source of new variation within existing
farmed stocks.
ACKNOWLEDGEMENTS
The authors would like to express their thanks to all colleagues who assisted
in the collection of the different species. These include Dr I. Payne and M r J.
Balarin, and particular gratitude is expressed to M r R. Haller of Baobab Farm,
Bamburi, Mombasa, Kenya, who supplied a number of species from his
collection (Table 1 ).
20
434
B. J. McANDREW AND K. C. MAJUMDAR
Dr E. Trewavas gave useful advice on suitable areas for collection of pure
species as well as making constructive comments on the manuscript. The
authors would also like to thank I. Kornfield, R. D. Ward, G. Hulata and
A. Abreu-Grois for their useful comments on an early draft of this manuscript.
The work was supported at Stirling by the Overseas Development
Administration. Kshitish Majumdar was also funded by the Commonwealth
Scholarship Commission.
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