interpretation of the life cycles of all remaining conchostracans. We need not postulate any changes in
the underlying genetic control of gender, but merely
that the phenotypic expression of the alternative alleles has been modified in different groups as a consequence of differences in their modes of reproduction.
Because of the recessiveness of the male-determining
factor, the genetic potentiality of males can be hidden
in lineages comprised of parthenogenetically reproducing females; a phenomenon that is illustrated in my
model of the evolution of the cyclestherian life cycle
(Fig. 4).
This evolutionary potential would not exist if
the sex-determining mechanisms of conchostracans
involved male, rather than female, heterogamety. Were
females the recessive gender, then parthenogenesis
would immediately eliminate the male-determining
factor. Furthermore, the conditions required for the
maintenance of androdioecy (Fig. 3) would be far more
stringent. In the Eulimnadia life cycle, for example,
both outcrossing and selfing modes of reproduction
are employed (Fig. 2B).1f reproduction was exclusively by selfing for a few successive generations, then the
male-determining allele would decrease in frequency
through time with the dynamical properties of a recessive lethal gene. The equilibrium condition would be
fixation on the dominant, female-determining, allele
(Fig. 3), but the rate of approach to equilibrium would
diminish as males become rarer. Periodic circumstances favoring outcrossing could restore the maledetermining allele to higher frequencies. If, instead, the
male-determining allele was dominant, then under selfing it would have the dynamical properties of a dominant lethal gene. Fixation on the female-determining
allele would occur in one generation and the androdioecious life cycle would be essentially incapable of
preserving male-determining factors.
Unisexuality in conchostracans has evolved multiple times (Fig. 1), and by several different routes from
different obligately sexual ancestors (Fig. 4). These
instances, therefore, represent examples of convergent
or parallel evolution rather than similarity by common
ancestry. This propensity toward unisexuality is also a
characteristic of other branchiopods. I have previously reviewed patterns of sex ratio variation and modes
of reproduction in the Notostraca (Sassaman, 1991),
many species of which have female-biased or unisexual populations. At least one species, Triops newberryi
(Packard), has an androdioecious life cycle, and pedigree analysis suggests that the male-determining allele
is recessive (Sassaman, 1991). The ancestral reproduc61
tive system of notostracans is not as readily deduced as
that in conchostracans, because there are only two genera and in each there is a mixture of obligately sexual
and unisexual life cycles. However, the inference that
unisexuality is a derived condition in the Conchostraca
means that it must have arisen from different ancestors
than the last common ancestor of conchostracans and
notostracans.
Cyclical parthenogenesis and obligate unisexuality also characterize many of the cladocerans (Hebert,
1987) and parthenogenesis is well established in populations of Artemia. Whether or not the findings of this
study can be applied toward explaining the evolution of
reproductive variation in these other groups is presently unknown. Aspects of sex determination in cladocerans (Hebert, 1987; Hob::ek & Larsson, 1990) appear
to differ from those inferred here for conchostracans,
and limited cytological evidence suggests that different families of Anostraca differ in their mechanisms
of sex determination (Bianchi Bullini et aI., 1968).
If so, then perhaps these branchiopods have been subject to different constraints, with different evolutionary
potentialities, in their evolution of modified life cycles
and alternative modes of reproduction.
Acknowledgments
Much of the renaissance of branchiopod biology is
due to the international cooperation and collegiality of
current students of this group. I am pleased to acknowledge the assistance of colleagues and friends in supplying samples, specimens, and supportive comments in
the course of this work. Among these are: Drs D. Belk
(San Antonio), L. Brendonck (Ghent), M. L. Fugate
(Riverside), J. King (Davis), J. Martin (Los Angeles), M. Simovich (San Diego), and W. Williams
(Adelaide). Experimental results reported here have
also been based on samples provided by G. Pratt and
D. Juliani. I thank Trudy Sassaman for helpful discussion and editorial suggestions on the manuscript. This
work has been supported by the Agricultural Experiment Station of the University of California and by
Academic Senate grants from UC Riverside.
the underlying genetic control of gender, but merely
that the phenotypic expression of the alternative alleles has been modified in different groups as a consequence of differences in their modes of reproduction.
Because of the recessiveness of the male-determining
factor, the genetic potentiality of males can be hidden
in lineages comprised of parthenogenetically reproducing females; a phenomenon that is illustrated in my
model of the evolution of the cyclestherian life cycle
(Fig. 4).
This evolutionary potential would not exist if
the sex-determining mechanisms of conchostracans
involved male, rather than female, heterogamety. Were
females the recessive gender, then parthenogenesis
would immediately eliminate the male-determining
factor. Furthermore, the conditions required for the
maintenance of androdioecy (Fig. 3) would be far more
stringent. In the Eulimnadia life cycle, for example,
both outcrossing and selfing modes of reproduction
are employed (Fig. 2B).1f reproduction was exclusively by selfing for a few successive generations, then the
male-determining allele would decrease in frequency
through time with the dynamical properties of a recessive lethal gene. The equilibrium condition would be
fixation on the dominant, female-determining, allele
(Fig. 3), but the rate of approach to equilibrium would
diminish as males become rarer. Periodic circumstances favoring outcrossing could restore the maledetermining allele to higher frequencies. If, instead, the
male-determining allele was dominant, then under selfing it would have the dynamical properties of a dominant lethal gene. Fixation on the female-determining
allele would occur in one generation and the androdioecious life cycle would be essentially incapable of
preserving male-determining factors.
Unisexuality in conchostracans has evolved multiple times (Fig. 1), and by several different routes from
different obligately sexual ancestors (Fig. 4). These
instances, therefore, represent examples of convergent
or parallel evolution rather than similarity by common
ancestry. This propensity toward unisexuality is also a
characteristic of other branchiopods. I have previously reviewed patterns of sex ratio variation and modes
of reproduction in the Notostraca (Sassaman, 1991),
many species of which have female-biased or unisexual populations. At least one species, Triops newberryi
(Packard), has an androdioecious life cycle, and pedigree analysis suggests that the male-determining allele
is recessive (Sassaman, 1991). The ancestral reproduc61
tive system of notostracans is not as readily deduced as
that in conchostracans, because there are only two genera and in each there is a mixture of obligately sexual
and unisexual life cycles. However, the inference that
unisexuality is a derived condition in the Conchostraca
means that it must have arisen from different ancestors
than the last common ancestor of conchostracans and
notostracans.
Cyclical parthenogenesis and obligate unisexuality also characterize many of the cladocerans (Hebert,
1987) and parthenogenesis is well established in populations of Artemia. Whether or not the findings of this
study can be applied toward explaining the evolution of
reproductive variation in these other groups is presently unknown. Aspects of sex determination in cladocerans (Hebert, 1987; Hob::ek & Larsson, 1990) appear
to differ from those inferred here for conchostracans,
and limited cytological evidence suggests that different families of Anostraca differ in their mechanisms
of sex determination (Bianchi Bullini et aI., 1968).
If so, then perhaps these branchiopods have been subject to different constraints, with different evolutionary
potentialities, in their evolution of modified life cycles
and alternative modes of reproduction.
Acknowledgments
Much of the renaissance of branchiopod biology is
due to the international cooperation and collegiality of
current students of this group. I am pleased to acknowledge the assistance of colleagues and friends in supplying samples, specimens, and supportive comments in
the course of this work. Among these are: Drs D. Belk
(San Antonio), L. Brendonck (Ghent), M. L. Fugate
(Riverside), J. King (Davis), J. Martin (Los Angeles), M. Simovich (San Diego), and W. Williams
(Adelaide). Experimental results reported here have
also been based on samples provided by G. Pratt and
D. Juliani. I thank Trudy Sassaman for helpful discussion and editorial suggestions on the manuscript. This
work has been supported by the Agricultural Experiment Station of the University of California and by
Academic Senate grants from UC Riverside.