Reproductive isolation and genetic differentiation in North American ...

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Figure 2. Genetic relatedness of Triops populations based on electrophoretic data. A. Phenogram generated by UPGMA clustering of Nei’s
unbiased distances. B. Dendogram generated by maximum likelihood neighbor joining of populations.
exhibit only female-biased or unisexual states (Table1),
and males rarely exceed 16% of the population.
Previous laboratory studies on several populations
of Triops newberryi have indicated that females from
both female-biased and from unisexual populations,
when reared entirely in isolation from hatching, nevertheless
produce viable cysts (Sassaman, 1991). Furthermore,
the pattern of appearance of males in clutches
produced during unisexual reproduction in this
species (Sassaman, 1991), by analogy with comparable
patterns in the conchostracan Eulimnadia texana
(Sassaman & Weeks, 1993), indicates that reproduction
is by selfing hermaphroditism.
Females from eastern populations of Triops longicaudatus
(those with sex ratios near equality) do not
produce viable cysts when reared in isolation.The cysts
that are produced are irregular in size, shape, and color;
they lack the normal extra-embryonic coatings; and
they do not hatch upon subsequent hydration. Limited
experiments with females from the Chinle Wash population
(the only female-biased population of T. longicaudatus
that we have studied), however, have yielded
some viable cysts produced by females reared in isolation.
Individualsfrom unisexual populations of T. longicaudatusproduce
viable cysts when reared in isolation
and histological evidence (Longhurst, 1955b; Akita,
1971; 1976) indicates that they are self-compatible hermaphrodites.
(For a review of the reproductive biology
of notostracans, see Sassaman (1991).)
We follow the terminology of Sassaman (1995:Figure
2) to denote the various populations of Triops in
terms of the reproductive characteristics of their life
cycles. Thus, bisexual populations (those containing
males) include both those with an obligately outcrossing
mode of reproduction (gonochoric) and those with a
mixed mating system involving outcrossing and facultative
selfing (androdioecious). Unisexual populations
of North American Triops all presumably reproduce
entirely by selfing.
Genetic correlates of reproductive biology
On the basis of allozyme differentiation (Figure 2) and
reproductive characteristics (Table 1), there are five
potentially distinct biological entities of North American
Triops: unisexual and androdioecious populations
of T. newberryi, and gonochoric, androdioecious and
unisexual populations of T. longicaudatus. InT. newberryi,
androdioecious and unisexual populations are
not sharply demarcated from each other; there is a continuous
gradation of sex ratios from 16% males to an
apparent absence of males (Table 1). Because of this
gradation, no population can be proven to lack males
completely; we can only estimate from our data the
maximum frequencies in which they might occur in