maturity completely in isolation. Of these, 7 were
crossed with males and 13 were kept isolated. All of
these females oviposited, brooded clutches of eggs
and released their eggs on the subsequent molt. Eggs
produced by unmated females were uniformly sterile;
none hatched and all of them disintegrated within two
cycles of drying and re-hydration. In contrast, over
90% of the eggs produced by mated females hatched
within two cycles of drying and re-hydration. Similar experiments with Caenestheria lutraria? and Leptestheria compleximanus have produced qualitatively similar results. In experiments with Cyzicus mexicanus, isolated females retained their oocytes within
the ovary and did not oviposit at all, whereas mated
females produced viable eggs. Field-collected Lynceus
gracilicornis and L. brachyurus did not oviposit after
being separated from males while those still in the
presence of males continued to produce eggs. Bishop (1967) indicated, without particulars, that Limnadia stanleyana is an obligately sexual species as well.
These studies on a variety of species with sex ratios
near unity indicate that such species are, indeed, obligately outcrossing.
In species characterized by female-biased sex
ratios, however, mating is not required. Belk (1972)
found that females of Eulimnadia antlei reared in isolation from female-biased populations nevertheless produced viable offspring and Strenth & Sissom (1975)
obtained the same result with E. texana. These findings have been confirmed for E. antlei by Sassaman
(1988) and for E. texana by Sassaman & Weeks (1993).
The latter study further demonstrated, using four different codominant genetic polymorphisms detectable
by protein electrophoresis, a binomial distribution of
offspring genotypes in clutches produced by heterozygous females. This genetic result implies that females
are self-compatible hermaphrodites. Further experiments, using genetic polymorph isms to verify outcrossing, showed that females also reproduce sexually. Indeed, in most experimental matings the offspring were a mixture of selfed and outcrossed individuals (Sassaman & Weeks, 1993). Thus females from
species in which males are present, but rare, are neither
obligately outcrossing nor obligately parthenogenetic.
They are, instead, facultatively capable of selfing in the
absence of males and of outcrossing in the presence of
males.
Unisexual species (i.e., those in which males are
totally absent, or absent in entire generations of the life
cycle) must be capable of unisexual reproduction; that
correspondence has been established for Cyzicus gyne51
cia (Mattox & Velardo, 1950), for Cyclestheria hislopi
populations (Sars, 1887; Paul & Nayar, 1977; Roessler
& Sanchez, 1986; Roessler, 1994c), for Eulimnadia
agassizii (Sassaman, unpubl. obs.), and for a yet undescribed species of Eulimnadia (cf: Belk, 1989) by
Stern & Stern (1971). To date, however, there has been
no Mendelian inheritance analysis in these species to
identify the genetic mechanism underlying this unisexual reproduction. One cytogenetic study of Limnadia
lenticularis, however, has established that reproduction involves suppression of the first meiotic division;
the egg pronucleus fuses with the first polar body prior
to the second meiotic division which proceeds normally (Zaffagnini, 1969).
These laboratory results on a variety of species substantiate the traditional interpretations of reproductive
mode from population sex ratio information. Species
with 1: 1 sex ratios are obligately sexual; species with
female-bias are facultatively capable of both unisexual
and bisexual reproduction; and all-female species are,
as expected, capable of unisexual reproduction.
Phylogeny of sex ratio
A useful method for investigating the evolutionary history of any attribute in a group of organisms, such as
unisexuality in Conchostraca, is to map the occurrence
of the attribute onto the phylogeny of the group in
question. To do so requires an explicit statement of the
phylogeny of the group. Figure 1 is a tree representing
such an hypothesis of the phylogeny of the Conchostraca.
The phylogenetic relationships among the five
extant families are generally accepted: Leptestheriidae
and Cyzicidae are closely related to each other and
together they most likely form the sister group to the
Limnadiidae. The Cyclestheriidae are usually viewed
as the next more-distant assemblage (Botnariuc & Vifia
Bayes, 1977) and the Lynceidae are generally regarded
as the most distant assemblage (Linder, 1945; Fryer,
1987).
Within the Lynceidae, zoogeographic distributions
and morphological characters tend to unite Paralimnetis and Lynceiopsis more closely than either to
Lynceus (Martin & Belk, 1988). The Cyclestheriidae
has long been viewed as a monotypic family containing only Cyclestheria hislopi. The recent addition of
Paracyclestheria sinensis (Shen & Dai, 1987) does not
materially alter the tree for Cyclestheriidae; still only
one topology is possible. In Cyzicidae, I have accepted the synonymy of Cyzicus and Caenestheriella as
crossed with males and 13 were kept isolated. All of
these females oviposited, brooded clutches of eggs
and released their eggs on the subsequent molt. Eggs
produced by unmated females were uniformly sterile;
none hatched and all of them disintegrated within two
cycles of drying and re-hydration. In contrast, over
90% of the eggs produced by mated females hatched
within two cycles of drying and re-hydration. Similar experiments with Caenestheria lutraria? and Leptestheria compleximanus have produced qualitatively similar results. In experiments with Cyzicus mexicanus, isolated females retained their oocytes within
the ovary and did not oviposit at all, whereas mated
females produced viable eggs. Field-collected Lynceus
gracilicornis and L. brachyurus did not oviposit after
being separated from males while those still in the
presence of males continued to produce eggs. Bishop (1967) indicated, without particulars, that Limnadia stanleyana is an obligately sexual species as well.
These studies on a variety of species with sex ratios
near unity indicate that such species are, indeed, obligately outcrossing.
In species characterized by female-biased sex
ratios, however, mating is not required. Belk (1972)
found that females of Eulimnadia antlei reared in isolation from female-biased populations nevertheless produced viable offspring and Strenth & Sissom (1975)
obtained the same result with E. texana. These findings have been confirmed for E. antlei by Sassaman
(1988) and for E. texana by Sassaman & Weeks (1993).
The latter study further demonstrated, using four different codominant genetic polymorphisms detectable
by protein electrophoresis, a binomial distribution of
offspring genotypes in clutches produced by heterozygous females. This genetic result implies that females
are self-compatible hermaphrodites. Further experiments, using genetic polymorph isms to verify outcrossing, showed that females also reproduce sexually. Indeed, in most experimental matings the offspring were a mixture of selfed and outcrossed individuals (Sassaman & Weeks, 1993). Thus females from
species in which males are present, but rare, are neither
obligately outcrossing nor obligately parthenogenetic.
They are, instead, facultatively capable of selfing in the
absence of males and of outcrossing in the presence of
males.
Unisexual species (i.e., those in which males are
totally absent, or absent in entire generations of the life
cycle) must be capable of unisexual reproduction; that
correspondence has been established for Cyzicus gyne51
cia (Mattox & Velardo, 1950), for Cyclestheria hislopi
populations (Sars, 1887; Paul & Nayar, 1977; Roessler
& Sanchez, 1986; Roessler, 1994c), for Eulimnadia
agassizii (Sassaman, unpubl. obs.), and for a yet undescribed species of Eulimnadia (cf: Belk, 1989) by
Stern & Stern (1971). To date, however, there has been
no Mendelian inheritance analysis in these species to
identify the genetic mechanism underlying this unisexual reproduction. One cytogenetic study of Limnadia
lenticularis, however, has established that reproduction involves suppression of the first meiotic division;
the egg pronucleus fuses with the first polar body prior
to the second meiotic division which proceeds normally (Zaffagnini, 1969).
These laboratory results on a variety of species substantiate the traditional interpretations of reproductive
mode from population sex ratio information. Species
with 1: 1 sex ratios are obligately sexual; species with
female-bias are facultatively capable of both unisexual
and bisexual reproduction; and all-female species are,
as expected, capable of unisexual reproduction.
Phylogeny of sex ratio
A useful method for investigating the evolutionary history of any attribute in a group of organisms, such as
unisexuality in Conchostraca, is to map the occurrence
of the attribute onto the phylogeny of the group in
question. To do so requires an explicit statement of the
phylogeny of the group. Figure 1 is a tree representing
such an hypothesis of the phylogeny of the Conchostraca.
The phylogenetic relationships among the five
extant families are generally accepted: Leptestheriidae
and Cyzicidae are closely related to each other and
together they most likely form the sister group to the
Limnadiidae. The Cyclestheriidae are usually viewed
as the next more-distant assemblage (Botnariuc & Vifia
Bayes, 1977) and the Lynceidae are generally regarded
as the most distant assemblage (Linder, 1945; Fryer,
1987).
Within the Lynceidae, zoogeographic distributions
and morphological characters tend to unite Paralimnetis and Lynceiopsis more closely than either to
Lynceus (Martin & Belk, 1988). The Cyclestheriidae
has long been viewed as a monotypic family containing only Cyclestheria hislopi. The recent addition of
Paracyclestheria sinensis (Shen & Dai, 1987) does not
materially alter the tree for Cyclestheriidae; still only
one topology is possible. In Cyzicidae, I have accepted the synonymy of Cyzicus and Caenestheriella as