Hydrobiologia 298: 45-65, 1995.
D. Belk, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
45
Sex determination and evolution of unisexuality in the Conchostraca
Clay Sassaman
Departmento/Biology, University o/California, Riverside, CA 92521, USA
Key words: Sex ratio, androdioecy, reproduction, genetics, parthenogenesis
Abstract
Field collected or laboratory-reared samples of 60 species of conchostracans (representing all extant genera)
indicate that males and females are equally common in most species. Deviations from this pattern occur in four
lineages. Cyzicus and Leptestheria each include at least one unisexual species; many species of Limnadiinae are
either unisexual or characterized by female-biased sex ratios; and Cyclestheriidae are either unisexual or express
males in the later generations of their life cycles. Laboratory studies indicate that species with sex ratios near
unity are gonochoric (obligately sexual), whereas females in species with female-biased sex ratios are capable
of both outcrossing and selfing modes of reproduction. Phylogenetic analysis of patterns of reproduction suggest
that sexual reproduction is the primitive condition. Genetic analysis of sexual species indicate that gender is
determined by one or a few genetic factors and that the male-determining allele is recessive. The inheritance
of gender in androdioecious species (where females are capable of self-fertilization) is similar to that in sexual
species. Androdioecy is likely to be the intermediate stage between obligately sexual reproduction and unisexuality
in the Limnadiinae. The phylogenetic distribution of sex ratio variation suggests that unisexuality in Cyzicidae,
Leptestheriidae, and Cyclestheriidae has arisen independently of that in the Limnadiinae and that these cases have
evolved by different evolutionary pathways.
Introduction
Organisms differ widely in the reproductive modes by
which genetic information is transmitted from one generation to the next (Bell, 1982). For those species in
which reproduction is sexual, considerable variation
also exists in the mechanisms that determine individual gender (Bull, 1983). Inquiry into the evolutionary
factors influencing reproductive modes has often proceeded apart from analysis of the mechanisms determining gender. However, these two features of organismal adaptation need not be as unconnected as they
might seem. Mechanisms of sex-determination, when
they are genetically based, are influenced by modes of
reproduction and can themselves be modified by selective processes operating on expressed genetic variability. Furthermore, factors that determine the expression
of sexual phenotypes may constrain the kinds of reproductive systems that can evolve. Finally, the genetic
determination of sex, when combined with differences
in reproductive modes, can provide an explicit mechanism for the sex ratio variation observed in natural and
laboratory popUlations.
My purpose here is to explore the relationship
between modes of reproduction and genetic mechanisms of sex determination with focus on the evolution
of unisexuality in conchostracans. The true evolutionary history of conchostracans and their modes of reproduction is buried in the past. This group is undoubtedly ancient (Linder, 1945; Tasch, 1969) and crucial
changes in life cycles and evolutionary genetics are
unlikely to be preserved in the fossil record. We are
left primarily with comparative methods to infer this
history. Credible models for the evolutionary processes
leading to unisexuality in conchostracans must define
precisely the initial (ancestral) condition, the current
condition, and also provide a plausible progression of
intermediate states between the two. Ideally, the models should also make specific biological predictions
that are testable by available techniques.
My approach is to review the limited literature on
these topics and to supplement these with data collect-
D. Belk, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
45
Sex determination and evolution of unisexuality in the Conchostraca
Clay Sassaman
Departmento/Biology, University o/California, Riverside, CA 92521, USA
Key words: Sex ratio, androdioecy, reproduction, genetics, parthenogenesis
Abstract
Field collected or laboratory-reared samples of 60 species of conchostracans (representing all extant genera)
indicate that males and females are equally common in most species. Deviations from this pattern occur in four
lineages. Cyzicus and Leptestheria each include at least one unisexual species; many species of Limnadiinae are
either unisexual or characterized by female-biased sex ratios; and Cyclestheriidae are either unisexual or express
males in the later generations of their life cycles. Laboratory studies indicate that species with sex ratios near
unity are gonochoric (obligately sexual), whereas females in species with female-biased sex ratios are capable
of both outcrossing and selfing modes of reproduction. Phylogenetic analysis of patterns of reproduction suggest
that sexual reproduction is the primitive condition. Genetic analysis of sexual species indicate that gender is
determined by one or a few genetic factors and that the male-determining allele is recessive. The inheritance
of gender in androdioecious species (where females are capable of self-fertilization) is similar to that in sexual
species. Androdioecy is likely to be the intermediate stage between obligately sexual reproduction and unisexuality
in the Limnadiinae. The phylogenetic distribution of sex ratio variation suggests that unisexuality in Cyzicidae,
Leptestheriidae, and Cyclestheriidae has arisen independently of that in the Limnadiinae and that these cases have
evolved by different evolutionary pathways.
Introduction
Organisms differ widely in the reproductive modes by
which genetic information is transmitted from one generation to the next (Bell, 1982). For those species in
which reproduction is sexual, considerable variation
also exists in the mechanisms that determine individual gender (Bull, 1983). Inquiry into the evolutionary
factors influencing reproductive modes has often proceeded apart from analysis of the mechanisms determining gender. However, these two features of organismal adaptation need not be as unconnected as they
might seem. Mechanisms of sex-determination, when
they are genetically based, are influenced by modes of
reproduction and can themselves be modified by selective processes operating on expressed genetic variability. Furthermore, factors that determine the expression
of sexual phenotypes may constrain the kinds of reproductive systems that can evolve. Finally, the genetic
determination of sex, when combined with differences
in reproductive modes, can provide an explicit mechanism for the sex ratio variation observed in natural and
laboratory popUlations.
My purpose here is to explore the relationship
between modes of reproduction and genetic mechanisms of sex determination with focus on the evolution
of unisexuality in conchostracans. The true evolutionary history of conchostracans and their modes of reproduction is buried in the past. This group is undoubtedly ancient (Linder, 1945; Tasch, 1969) and crucial
changes in life cycles and evolutionary genetics are
unlikely to be preserved in the fossil record. We are
left primarily with comparative methods to infer this
history. Credible models for the evolutionary processes
leading to unisexuality in conchostracans must define
precisely the initial (ancestral) condition, the current
condition, and also provide a plausible progression of
intermediate states between the two. Ideally, the models should also make specific biological predictions
that are testable by available techniques.
My approach is to review the limited literature on
these topics and to supplement these with data collect-