AMER. ZOOI... 19:729-737 (1979).
Evolution and Ecology of Parthenogenesis in Earthworms
JOHN JAENIKE AND ROBERT K. SELANDER
Department of Biology, University of Rochester, Rochester, New York 14627
SYNOPSIS. A model for the origin of parthenogenesis in hermaphrodites is developed. If a
dominant mutation causing pdrthenogenetic development of eggs without affecting
meiotic production of sperm arises, the parthenogens will increase in frequency to fixation.
Concomitantly, there is selection for reallocation of resources from male- to female-related
functions in both parthenogenetic and sexual individuals. Occasional fertilization of unreduced eggs may produce polyploid clones. Both the loss of male-related structures and
polyploidy are common in parthenogenetic earthworms. Parthenogenesis should be
favored in patchy and temporally unstable habitats, in which r-selection may be expected,
because it facilitates colonization and rapid population growth, and because selection by
the biotic component of the environment presumably is reduced. Parthenogenetic earthworms commonly occur in decaying logs, leaf litter, and the upper, organic layers of the
soil, whereas sexual species more often inhabit the deeper, more stable soil horizons.
Long-term persistence of clones depends on their ability to survive and reproduce under
a variety of environmental conditions. It is proposed that successful clones possess "general
purpose" genotypes that allow persistence in spite of temporal changes and facilitate
active dispersal through heterogeneous environments between patches of prime habitat.
Two common clones of the parthenogenetic earthworm Octolasion tyrtaeum seem to possess
general purpose genotypes, as they occur in a wide variety of soil and habitat types and
are geographically widespread.
INTRODUCTION
In recent years, increasing attention has
been directed to variation in mode of reproduction among animal species, with the
primary objective of determining the relative advantages of sex and parthenogenesis.
For a balanced review of recent work on
breeding systems and the plethora of adaptive interpretations now current, see Maynard Smith (1978), who also presents a
skeleton key to the literature.
Although studies of isolated, unusual
cases of parthenogenesis in normally sexual
groups of organisms may contribute to an
understanding of the evolution of breeding
systems, we believe that a better approach is
to work with taxonomic groups in which
parthenogenesis has arisen independently
in a large number of species. If a sufficient
number of species is examined, the study
Present address of J. Jaenike is: Department of
Biological Sciences, State University of New York,
Binghamton, New York 13901.
Research supported by NIH Grant GM-22126 and
NSF Grant BMS 74-24108.
becomes grossly analogous to. a controlled
experiment. The primary advantage of this
approach is that the results of historical accident are less likely to obscure adaptive
trends related to factors of contemporary
environments. Terrestrial earthworms
(megadrile oligochaetes) provide excellent
material for an analysis of breeding systems, since many of the species are parthenogenetic.
We are here concerned primarily with
the North American representatives of the
family Lumbricidae, among which 17 of 33
species reproduce primarily or exclusively
by parthenogenesis (Table 1). Our objectives are to examine several problems related to the evolution and adaptive significance of parthenogenesis, including the
development of polyploid series of clones in
some species; clonal variation in the extent
of reduction of male reproductive structures; ecological differences between parthenogenetic and amphimictic species; and
the nature of successful parthenogenetic
genotypes. We first develop a model for the
spread of a new mutation which causes parthenogenetic development of eggs in a
729
730
JOHN JAENIKE AND R. K. SELANDER
TABLE 1. Reproduction by North American
Lumbricidae*
Species"
Allolobophora chlorotica
Aporrectodea limicola
longa
rosea
trapezoides
tuberculata
turgida
Bimastos beddardi
gieseleri
heimburgeri
longicinctus
palustris
parvus
tumidus
welchi
zeteki
Dendrobaena octaedra
Dendrodrilus rubidus
Eisenia foetida
hortensis
zebra
Eiseniella tetraedra
Eisenoides caroltnensis
Ib'nnbergi
Lumbricus castaneus
Jestwus
rtibellus
terrestris
eiseni
Murchieona muldali
Octolasion cyaneum
tyrtaeum
Salchellius mammale
a
b
c
d
Mode of
reproduction 0
A
A
A
P
P
A
A
P
P
P
P
P
P
P
P
P
P
P
A
A
A
P
A
A
A
A
A
A
P
A
P
P
A
Origin11
E
E
E
E
E
E
E
N
N
N
N
N
N
N
N
N
E
E
E
E
E
E
N
N
E
E
E
E
E
E
E
E
E
From Reynolds (1974a).
Nomenclature based on Gates (1978).
A = Amphimictic
P = Parthenogenetic
E = Exotic, introduced species
N = Nearctic, native species
hermaphroditic population. As the frequency of parthenogens increases, there is
increasing selection for reallocation of resources from male- to female-related reproductive functions. This simple model
can also account for the existence of both
"parthenogenetic polymorphism" (the loss,
to varying degree, of male-related functions) and polyploid series within taxonomically denned species.
If the origin of parthenogenetic reproduction in earthworms is not an exceedingly rare event, why, then, do not all
species reproduce in this manner? A provisional answer may be found by considering
the sorts of habitats generally occupied by
parthenogenetic and sexual species. The
evidence indicates that parthenogenetic
earthworms commonly occur in ephemeral
or unstable habitats, in which r-selection
may be expected, whereas sexual species
tend to inhabit more stable environmental
situations, where /(-selection may be more
important. Finally, because of the nature of
the habitats they occupy and the absence of
genetic variation in their offspring, successful clones of parthenogenetic species may
be expected to possess "general purpose"
genotypes. Evidence that this is indeed so in
at least one parthenogenetic species is presented.
SELECTION FOR PARTHENOGENESIS IN A
HERMAPHRODITIC POPULATION
In this section, we consider the fate of a
new dominant mutant, P, arising in a diploid, sexual hermaphroditic species. Bearers of the mutant allele (genotype +/P)
produce eggs parthenogentically and sperm
by normal meiosis. (Maynard Smith [1978]
mentioned this type of mutation but did not
explore its consequences in detail.) All eggs
produced by +/P individuals are of the
genotype +/P, because they are formed
apomictically, while half their sperm carry
the + allele and half the P allele. (Note that
P/P genotypes are not produced.) We assume that the species reproduces by reciprocal fertilization, as sexual earthworms do.
In a sexual hermaphroditic population,
each individual fertilizes, on the average, as
many eggs as it produces. Let us define the
average fitness in the population as 1; that
is, each individual produces one egg and
fertilizes one egg. We wish to study the
population genetics of mutant P in this
population. Assume that the probability of
finding a mate is so high that all of an individual's eggs are fertilized. (Because this
assumption minimizes the potential advantage of parthenogenesis, our results are
conservative.) Regardless of the frequency
off, the average fitness in the population
remains 1, since each individual still produces the same number of eggs. Let q be the
frequency of +/P individuals. If mating occurs randomly, then for individuals of +/P
genotype, a fraction q of the matings will be
731
PARTHENOGENESIS IN EARTHWORMS
with + /P types, and 1-qwith + / + . In matings of the former type, both individuals
produce one +/P egg but fertilize no eggs
with their sperm. Hence, each parent produces, on average, one +/P offspring. In
matings with +/ + , however, the +/P individual produces one +/P egg parthenogenetically and fertilizes one egg of the
+ / + parent, an average of one-half of
which will yield +/P offspring. In this type
of mating, the +/P genotype is represented
in 3/2 offspring, which is the relative fitness
of a +/P individual when mated to one of
genotype +/ + . Since the average fitness of
the population is always 1, the fitness of
+/P types is
TABLE 2. Polyploid series in parthenogenetic
Lumbricidae."
Species
Aporrectodea trapezoides
Aporrectodea rosea
Dendrobaena octaedra
Dendrodrilus rubidus
Octolasion tyrtaeum
a
= (3/2)This takes into account the relative frequency of the two types of mating and the
number of +/P offspring per +/P parent in
each.
If generations are discrete, the frequency
of +/P individuals at generation t + l is <?,+,
= W{+iP)q, = (3/2)<7 - (1/2),/2. The change in
the frequency of genotype +/P from one
generation to the next, Aq, is equal to
(1/2)^(1 — q), which is positive for all values
of q between 0 and 1. Thus the frequency
of parthenogenetic individuals in the population increases to fixation.
If parthenogenesis can arise by the mechanism considered here, then a new clone
is generated each time an egg is fertilized
by a P-bearing sperm; and a large number of genetically diverse clones will be
produced, providing a substantial basis for
the action of natural selection. Clones that
eventually become numerically dominant
may be expected to be more highly adapted
than those derived from a gonochoristic
species, in which new mutations arise only
by mutation.
A consequence of the dominance of the/ 3
allele is that fertilization of a +/P egg can
produce a clone of higher ploidy level. Continuation of this process could lead to the
creation of polyploid series within one
parthenogenetic "species." Although cytological studies of earthworms have not been
extensive, several such series have been
found (Table 2). In some of these species
N umber of
chromosomes
2n
3n
3n
5n
6n
6n
In
2n
4n
6n
2n
3n
4n
=36
= 54
= 54
=90
= 108
= 108
= 124
= 34
= 68
= 102
= 38
=54
= 72
From Muldal (1952); Omodeo (1952, 1955).
both diploid and polyploid forms have
been identified. (Polyploidy is rare or absent in amphimictic lumbricids; Muldal
[1952], Omodeo [1952, 1955].) If our
model is realistic, we may conclude that
the origin of parthenogenesis precedes
that of polyploidy.
REALLOCATION OF RESOURCES IN
PARTHENOGENETIC INDIVIDUALS
In the model considered in the previous
section, we assumed that individuals invest
the same amount of resources in male- and
female-related reproductive functions as
did those in the ancestral, entirely amphimictic population. But intuitively it
seems that the benefit of investment in
male-related functions would decrease as
the frequency of parthenogenetic (+/P)
individuals increases. Here we present
a quantitative model confirming this
conclusion.
Assume that there is some quantity of
resources (Rm) that may be invested in
either male or female functions in a hermaphrodite, but which in individuals of an
amphimictic population is primarily allocated to male functions, which include
sperm production, storage, and transfer.
We wish to know how many eggs (A's) an
individual can potentially fertilize in its
lifetime as a function of the proportional
investment of these resources in male functions (Im). The value of Ns will depend upon
(A. trnpezoides, D. rubidus, and O. tyrtaeum), such factors as longevity and frequency of
732
JOHN JAENIKE AND R. K. SELANDER
mating. If investment is less than the
minimum needed to build functional male
reproductive structures, then Ns will be
zero. As investment increases beyond this
minimum, Ns rises with increased sperm
production and the development of additional sets of male organs; and it finally
plateaus at a value corresponding to the
maximum number of eggs that can be fertilized in these matings (Fig. 1). In sparse
populations, where mating is infrequent,
and in short-lived organisms, the asymptotic value of Ns should be relatively low.
Now suppose that some or all of Rm is
invested in egg-producing functions. Because earthworms are hermaphroditic, the
egg-producing machinery is already present, and these resources may be channeled
either directly into eggs or, perhaps, into
additional egg-producing organs. Alternatively, egg production may be increased
indirectly through higher probability of
survival as investment in male functions
is reduced. In either case, we expect the
number of eggs produced as a result of
reallocation of resources (Ne) to rise more
or less linearly with the amount of reallocation. (For + / + individuals, this assumes
that all eggs will be fertilized.) The slope of
this line is directly proportional to the efficiency of conversion of these resources
from sperm- to egg-producing functions.
For example, if eggs require large amounts
of some limiting nutrient which is scarce in
male structures and sperm, the efficiency of
conversion will be low. In Figure l,Ne and
Ns are plotted as functions of Im.
For each value of/m, there is an associated
potential number of eggs fertilized and
number of eggs produced (Ne). Therefore,
we can construct a fitness set whose axes
are Ne and Ns, with points corresponding to various values of Im (Fig. 2). (Charnov et al. [1976] used a similar construct
to model the evolution of hermaphroditism from dioecy.) Next we need an
adaptive function to determine the optimal
value of Im as a function of the frequency of
+/P individuals. We consider this first for
the parthenogenetic (+/P) segment of the
population. Because the eggs of these individuals are produced parthenogenetically,
all Ne of them will be of genotype +/P. The
actual number of eggs fertilized will be less
than the potential number (Ns), because
only matings with +/+ individuals yield
offspring via sperm. If mating is random,
only ( 1 - q)Ns eggs will be fertilized, onehalf of which will be of genotype +/P. The
adaptive function maximizing the number
A/c
<7=.5
N
e
FIG. 1. Number of offspring produced via eggs (Me)
and sperm (A',) as a function of the proportional investment of resources in male-related functions (Im).
Initial investment in male structures may be large (a) or
small (b), and efficiency of conversion of resources from
male to female functions may be high (c) or low (d).
FIG. 2. Fitness set based on curves a and c of Figure
I, where for each value of Im there is a corresponding
pair of iV,, and Ns. The adaptive functions for parthenogens at several frequencies (q) are shown. For parthenogens, the adaptive function to be maximized is
W<+/n =Ne + (1/2)^,(1 - q), as plotted here forsexual
individuals, it is W,+l+) = (1/2) (1 - q) (A'e + A',) +
PARTHENOGENESIS IN EARTHWORMS
733
of +/P offspring through eggs and sperm is as a function of q, in order to determine
simply the sum of their separate contribu- what is potentially available for the creation
of new clones. Consider the two types of
tions, namely
mating from the point of view of a +/ +
W(+IP)=Ne+(\/2
N f (l
individual, and the payoff in + / + offspring
from its own eggs and from those fertilized
or
by its sperm. A fraction 1 — q of the matings
2[W
-N
]
=
t+IP)
e
are with other amphimictic individuals,
1- q
yielding Ne +/+ offspring via its own eggs
Initially, when there are virtually no par- and Ns offspring through eggs fertilized by
thenogens in the population (q = 0), the its sperm, all of which have a second +/+
slope of the adaptive function is - 2 . As the parent. The contribution of one + / + indifrequency of +/P individuals increases, the vidual to these offspring is therefore oneslope becomes steeper, and at q = 1 it is half of the total. The remaining fraction, q,
infinite (Fig. 2). In essence, this means that of its matings are with parthenogenetic +/P
as the frequency of the parthenogens in- individuals. In this case, the +/+ parent
creases, optimal investment in male func- fertilizes no eggs, and only (l/2)iVe q of its
tions drops. Depending on the shape of the own eggs yield +/+ genotypes, an equivafitness set, this optimal investment de- lent number, of course, producing +/P
creases slowly as q initially increases and clones. The genotypes which will be most fit
then jumps discontinuously to zero as q in thesexual part of the population are those
reaches some critical value. In this case, we which maximize the adaptive function
would first expect to see selection for clones
W
= (1/2)
(I-q)(Ne+Ns)
with moderate investment in male func(+/+)
tions and, later, selection for those with
+ (l/2)qNe ,
none. However, if there are no clones with
/ m = 0, and if reallocation can proceed only or
-Ne
by small steps, evolutionary changes in the
l-q
population will be constrained to occur
along the fitness set line. In this case, Im is for the fitness set based on N and N (Fig.
e
s
gradually reduced as q increases, and it does 2). Atq = 0, the slope of the adaptive
funcnot drop discontinuously to zero. Eventu- tion is —1; and as the frequency of partheally, however, the adaptive function be- nogens increases, the slope steepens, finally
comes tangent to the point of inflection in becoming infinite at q = 1. In other words,
the fitness set, whereafter Im gradually de- the optimal investment in male functions
clines to zero, independently of changes in q. decreases in the sexual component of the
Selection for any trait in the partheno- population as the frequency of parthegens, such as reallocation of resources, de- nogens increases. Because this optimal
pends on variation among clones. New +/P investment also decreases in the partheclones can be generated by mutation within nogens as q increases, it follows that the
existing clones and through fertilization of sexually reproducing part of the popula+-bearing eggs by sperm carrying the P tion is continually generating, through
allele. The development of genetic varia- recombination and selection, new arrays of
tion within the +/P segment of the popula- Im suitable for the parthenogens.
tion must initially be much slower by the
Loss of male reproductive structures and
former process. Accordingly, in generating variation among "morphs" in the degree of
new clones with more nearly optimal alloca- this loss have been studied in several parthetion of reproductive resources, reliance is nogenetic lumbricids. The latter phenommade primarily on the variance of Im within enon is referred to as "parthenogenetic
the +/+, amphimictic section of the popu- polymorphism." The following discuslation.
sion is based on the extensive and valuable
Now we wish to see how the optimal lm for work of Gates (1972, 1973, \974a,b,c,
sexually reproducing individuals changes 1977). The species Octolasion cyaneum and
734
JOHN JAENIKE AND R. K. SELANDER
O. tyrtaeum show relatively little loss of male sperm. In other morphs, however, some or
structures. Various glands associated with all of the testes, seminal vesicles, spercopulation are vestigial in some morphs, mathecae, and accessory glands are rebut the seminal vesicles, in which sperm duced or absent. Finally, some morphs ofD.
mature, are of medium size; and some indi- octaedra and E. tetraedra are hypergynous,
viduals apparently produce normal sperm. that is, they possess extra pairs of ovaries.
In addition, Gates (1972) has observed This suggests that in addition to being concopulation between parthenogenetic indi- served through the loss of male structures,
viduals. These facts suggest that parthe- resources may also be reallocated to the
nogenesis has evolved rather recently in production of additional female organs.
these species. On the other hand, two com- Whether the loss of male structures and
mon clones of Octolasion tyrtaeum found parthenogenetic polymorphism have rein eastern North America differ elec- sulted from mutations within clones or
trophoretically at four of 10 loci studied, from the de nova creation of new clones is
suggesting a rather ancient origin (Jaenike unknown.
etal., 1980). These seemingly contradictory
observations may be reconciled if partheHABITATS OF SEXUAL AND PARTHENOGENETIC
nogenesis is of multiple origin, with clonal
EARTHWORMS
diversity perhaps arising by the mechanism
we have described.
If sexual and parthenogenetic individuLoss of male structures has proceeded als have similar survival rates and fertilities,
somewhat further in Aporrectodea trapezoidesthen, as shown above and by others
and Bimastos parvus, in which some or all of (Maynard Smith, 1971; Williams, 1975), the
the seminal veiscles are small or rudimen- frequency of genes determining parthetary, and sperm are rarely, if ever, pro- nogenesis in a population will increase to
fixation. This brings us to the perennial
duced.
Extensive parthenogenetic polymorph- problem of why sex is maintained within
ism has been described in Dendrobaena oc- populations. Following Williams' (1975)
taedra, Dendrodrilus rubidus, Aporrectodea suggestion that attention should focus on
rosea, and Eiseniella tetraedra. In each of possible short-term advantages of sex, we
these species, some morphs have under- believe that insight is provided by examingone relatively little loss of male structures ing ecological differences between sexual
and may even produce apparently normal and parthenogenetic species of earthI ABI.K 3. Common habitats oj North American Lumbricidae. *
Reproduction
Species
Habitat
Parthenogenetic
Aporrectodea rosea
Aporrectodea trapezoides
Bimastos spp.
Dendrobaena octaedra
Middle to upper soil layers
Middle to upper soil layers
Decaying logs and litter
Litter and surface soil layers; often in areas of high organic
content
Decaying logs
Wet areas and damp litter; rarely in soil
Stream beds and other moist areas
Litter and upper soil layers
Upper layers of moist, organically rich soil
Deep soil layers
Middle to upper soil layers
Deep soil layers
L'pper soil layers
Lppersoil layers
Deep soil layers
Amphimictic
a
Dendrodrilus rubidus
Eiseniella tetraedra
Octolasion cyaneum
Octolasion tyrtaeum
Allolobophora chlorotica
Aporrectodea longa
Aporrectodea tuberculata
Aporrectodea turgida
Lumbricus castaneus
Lumbriciis rubellus
Lumbricus terrestris
From Nordstrom and Rundgren (1973); Reynolds (1974*. 1976, 1977); Reynolds et al. (1974); Rundgren
(1975).
PARTHENOGENESIS IN EARTHWORMS
worms. The habitats of lumbricids in North
America for which there is at least a modicum of information are presented in Table
3. While all the sexual species inhabit soil at
some depth, many of the parthenogens
dwell primarily in litter and decaying logs
on the surface.
Why are the litter and decaying log
niches occupied solely by parthenogenetic
species, and why are the sexual species not
entirely displaced from the soil by parthenogens? Provisional answers to these questions can be couched in terms of r- and
/^-selection. Logs in an appropriate condition of decay for habitation by earthworms
are a spatially patchy and ephemeral resource, as are heavy concentrations of leaf
litter and debris. These are habitats in
which r-selection may be expected to predominate. Successful exploitation of these
resources requires high colonizing ability
and a potential for rapid population
growth, as they represent a bonanza to the
first colonists. Parthenogens should be
more successful colonists than sexuals because only one individual is necessary
(Tomlinson, 1966) and they have a greater
intrinsic rate of increase (Williams, 1975).
Parthenogenetic earthworms also commonly inhabit the superficial layers of soil,
which are much less stable temporally in
temperature and moisture content than are
deeper soil horizons (Nordstrom and Rundgren, 1974). Accordingly, earthworms
dwelling near the surface should be more
affected by vicissitudes of the climate, such
as high temperature and drought. If these
cause high density-independent mortality,
populations may often be reduced below
carrying capacity and occasionally wiped
out altogether. Earthworms obliged to
dwell near the surface of the soil, because
of inability to burrow deeply, for instance,
should ber-selected if their populations experience drastic fluctuations in size.
As shown above and by Maynard Smith
(1971), parthenogens should replace sexually reproducing forms in any habitat, given
equal fertilities and survival rates. Why
then are sexual earthworms common in the
soil niche? While decaying logs and concentrations of litter are spatially and temporally patchy, the soil has much greater
735
spatial continuity and temporal stability,
properties which probably increase with
soil depth. This is the type of habitat in
which /^-selection may be expected. A
number of workers (Levin, 1975; Selander
and Hudson, 1976; Glesener and Tilman,
1978; Jaenike, 1978; Hamilton etal., 1979)
believe that selection by the biotic component of the environment (predators, pathogens, etc.) in stable habitats favors sexual reproduction. In essence, if the various genotypes of a species are differentially
affected by other species or genotypes, the
genetic and numerical reactivity of the biotic environment puts a premium on the generation of rare or novel genotypes. Sex and
recombination are the means by which this
is achieved. Parthenogenetic clones, with
their inability to produce genetically heterogeneous progeny, may be devastated
by a biotic selective agent. The continuity
and stability of the soil may allow species of
earthworms occupying this niche to maintain relatively stable and uniform population densities. They may therefore be predictable resources for their parasites, etc.,
which may then exert considerable selective
pressure. Sexual reproduction in earthworms, by producing genetically diverse
descendants, lowers the probability that
they will suffer heavily from parasitism.
Parthenogenetic clones could conceivably
be wiped out in these stable habitats.
Parthenogenesis is not selected against in
the decaying log and litter niches precisely
because of their patchy, ephemeral nature.
To exert a genotype-specific selective influence on earthworms, parasites must be able
to track their hosts numerically and genetically. The patchy nature of the parthenogens' habitat renders this unlikely and
means that mortality from parasitism will
be more or less random with respect to host
genotype. Therefore, there is no selective
advantage to producing genetically variable
offspring, at least with respect to the biotic
environment. Earthworms living in patchy
habitats can enjoy the advantages of parthenogenesis—superior colonizing ability
and a high intrinsic rate of increase — without paying the penalty of increased mortality and reduced fertility imposed by
other species.
736
JOHN JAENIKE AND R. K. SELANDER
PARTHENOGENETIC GENOTYPES
Because parthenogens do not produce
genetically heterogeneous offspring, some
other means of adapting to changes in the
environment from one generation to the
next is necessary for persistence. A measure
of the long-term success of a clone is the
geometric mean of its fitness over generations, a quantity which for a given arithmetic mean varies inversely with temporal
variance in fitness. Clones which are superior in the long run should be those whose
fitness is relatively unaffected by environmental changes. They should be characterized by ecologically broad-niched and physiologically and developmentally flexible
"general purpose" genotypes, as Baker
(1965) has termed them. The possession of
such genotypes also allows the simultaneous
use by members of a clone of a variety of
habitats in a spatially heterogeneous environment. For slow-moving organisms,
such as earthworms, that disperse by active
passage through the environment, this flexibility in turn facilitates dispersal among
patchily distributed prime habitats. Finally,
the ability to survive and reproduce under a
wide range of conditions should lead eventually to the occupation of broad geographic
areas by successful clones.
We have studied the ecology of the
parthenogenetic lumbricid Octolasion tyrtaeum in New York, North Carolina, and
Tennessee (Jaenike et al., 1980). Among
over 2,000 individuals studied electrophoretically, we have identified eight genetically distinct clones, two of which, A and
B, are overwhelmingly dominant. In North
Carolina and Tennessee, where most of our
collections were made, these two clones are
ecologically broad-niched. Ten habitat
types, ranging from stream banks and fields
to various forest types, were represented
among the 64 localities where O. tyrtaeum
was collected. Clone A was found in nine of
these habitats and clone B in all 10. The pH
of the soil in which this species was collected
ranged from 4.3 to 8.1; those of soils where
clones A and B occurred were 4.3 to 7.5 and
4.7 to 8.1, or 84% and 89% of the total pH
range, respectively. The pH ranges of soils
inhabited by these two clones are as great or
greater than those of many sexual species of
earthworms (Gates, 1972). Finally, O. tyrtaeum was collected from soils whose textures ranged from 26% to 97% sand. Clone
A occupied soils spanning the entire range,
while clone B was found in soils of 26% to
96% sand content, or 99% of the total
range. With respect to these three environmental variables, habitat type, soil pH,
and soil texture, the two dominant clones of
O. tyrtaeum are very broad-niched, and, accordingly, may be said to possess general
purpose genotypes. Their ecological generalism has also allowed clones A and B
to occupy relatively wide geographic
ranges; they are the only clones of O. tyrtaeum recorded in New York.
If, for the reasons set out above, parthenogenetic clones tend to have general purpose genotypes, it follows that some clones
may be ecologically similar and experience
intense competition in areas where they
occur together. Stable coexistence would
then be most likely only for ecologically distinct clones, which, therefore, would not
have fundamental niches as broad as those
envisioned for clones possessing general
purpose genotypes. Consequently, such
clones could not be as opportunistic in their
exploitation of the environment. But if the
habitats occupied by a species were sufficiently patchy and ephemeral, co-occurrence of different clones in any one area
would be infrequent and transitory. Under
such conditions, interclonal competition
would be of little selective importance,
and the most successful clones would be
those that are ecologically broad-niched. In
general, the degree of ecological differentiation among clones should be positively
correlated with the extent with which they
coexist.
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