Sex determination
In contrast to patterns of variation in modes of reproduction, less is known about the underlying mechanisms of sex determination in conchostracans. Population sex ratio is useful in determining whether or not a
particular species reproduces sexually, but is relatively
uninformative about the nature of gender determination. Gender may be determined by inductive environmental conditions, by diffuse interactions in a polygenic system, by a single factor (gene or chromosome)
with major effect, or by an interaction between environmental and intrinsic influences.
In the species of Conchostraca that I have examined to date, sex appears to be genetically determined
by a single factor with major effect. Furthermore, the
genetic basis of sex determination in obligately sexual species, such as Eocyzicus, is fundamentally the
same as that in facultatively unisexual species, such
as Eulimnadia. Finally, the genetic determination of
sex in these species, when combined with differences
in reproductive modes, provides an explicit mechanism for the sex ratio variation observed in natural
and laboratory populations (Table 1). I will describe
these findings about sex determination in two circumstances where it has been analyzed experimentally and
then extend these observations to the interpretation of
other situations that have not yet been studied in the
laboratory.
Obligate sexuality
Although population sex ratios indicate little directly
about sex determination, the ratios of offspring sexes
in individual matings is informative. For example, if
sex is determined by a single factor carried in heterozygous (or hemizygous) condition in one of the parents,
then all crosses are expected to produce sex ratios of 1
male: 1 female. If sex is determined by mUltiple polymorphic factors, or there are environmental influences,
then clutch sex ratios may vary significantly among
crosses from different parents.
Evidence from three species of obligately sexual
clam shrimps support the view of single-factor inheritance. I have reared offspring from 30 single-pair matings of Eocyzicus concavus; only one clutch deviated
significantly from a 1: 1 sex ratio (atP<0.05), a result to
be expected by chance alone. The aggregate output of
these 30 crosses was 1225 males and 1257 females, or
49.4% males. Limited experiments with Caenestheria
53
lutraria? (3 clutches) and Leptestheria compleximanus
(1 clutch) have the same male:female equality.
Clutch sex ratios, while supporting genetic determination, do not prove the case. Nor do they indicate which parent is carrying the alternative factors
that might be responsible for gender specification.
There are several principal methods for investigating
these aspects of sex determination in obligately sexual species: analysis of offspring sex ratios in progenies
of gender-manipulated individuals, sexual dimorphism
of karyotype, and inheritance analysis of sex-linked
genes. The first method is not easily applied to branchiopods because of their relatively short life cycle
and rather delicate constitution but has been useful
in investigating other groups of animals (Humphrey,
1945; Katakura, 1984). The second has been applied
to some Anostraca (Bianchi Bullini et aI., 1968), leading to the suggestion that in Artemia females are heterogametic whereas in some other forms the males are
hemigametic. Chromosomes of Conchostraca are quite
small (Zaffagnini, 1969), and methods have yet to be
developed for the reliable analysis of their karyotypes.
I have found the third method, however, to be quite
useful in the analysis of sex determining mechanisms
of clam shrimps, although it has to date been applied
only to Eocyzicus concavus.
The joint inheritance of offspring sex and genotype
at an enzyme locus with electrophoretically detectable
alleles in E. concavus demonstrates female heterogamety (Sassaman, 1990; unpubl. obs.). The locus (Idh2, encoding the enzyme NADP-dependent isocitrate
dehydrogenase) is in double-copy in both sexes, but
the inheritance of alleles is partially sex-linked. Backcrosses of heterozygous males yield progeny genotype
distributions that are random with respect to offspring
sex; however, backcrosses of heterozygous females
yield a strong (nearly perfect) correlation between offspring sex and offspring genotype. This pattern of
inheritance is phase-dependent; the specific association between offspring sex and genotype can be manipulated by changing the origin of the alleles in the tested
female. The overall pattern of inheritance is consistent with female heterogamety and tight linkage of the
marker locus (Idh-2) with the region of the genome
carrying sex determining elements. (Limited data also
indicate tight linkage of another enzyme-encoding
locus - Fum, fumarate dehydratase - to both the sex
determining locus and Idh-2.) This pattern of partially sex-linked inheritance of enzyme-encoding genes
in Eocyzicus is similar to that previously described for
In contrast to patterns of variation in modes of reproduction, less is known about the underlying mechanisms of sex determination in conchostracans. Population sex ratio is useful in determining whether or not a
particular species reproduces sexually, but is relatively
uninformative about the nature of gender determination. Gender may be determined by inductive environmental conditions, by diffuse interactions in a polygenic system, by a single factor (gene or chromosome)
with major effect, or by an interaction between environmental and intrinsic influences.
In the species of Conchostraca that I have examined to date, sex appears to be genetically determined
by a single factor with major effect. Furthermore, the
genetic basis of sex determination in obligately sexual species, such as Eocyzicus, is fundamentally the
same as that in facultatively unisexual species, such
as Eulimnadia. Finally, the genetic determination of
sex in these species, when combined with differences
in reproductive modes, provides an explicit mechanism for the sex ratio variation observed in natural
and laboratory populations (Table 1). I will describe
these findings about sex determination in two circumstances where it has been analyzed experimentally and
then extend these observations to the interpretation of
other situations that have not yet been studied in the
laboratory.
Obligate sexuality
Although population sex ratios indicate little directly
about sex determination, the ratios of offspring sexes
in individual matings is informative. For example, if
sex is determined by a single factor carried in heterozygous (or hemizygous) condition in one of the parents,
then all crosses are expected to produce sex ratios of 1
male: 1 female. If sex is determined by mUltiple polymorphic factors, or there are environmental influences,
then clutch sex ratios may vary significantly among
crosses from different parents.
Evidence from three species of obligately sexual
clam shrimps support the view of single-factor inheritance. I have reared offspring from 30 single-pair matings of Eocyzicus concavus; only one clutch deviated
significantly from a 1: 1 sex ratio (atP<0.05), a result to
be expected by chance alone. The aggregate output of
these 30 crosses was 1225 males and 1257 females, or
49.4% males. Limited experiments with Caenestheria
53
lutraria? (3 clutches) and Leptestheria compleximanus
(1 clutch) have the same male:female equality.
Clutch sex ratios, while supporting genetic determination, do not prove the case. Nor do they indicate which parent is carrying the alternative factors
that might be responsible for gender specification.
There are several principal methods for investigating
these aspects of sex determination in obligately sexual species: analysis of offspring sex ratios in progenies
of gender-manipulated individuals, sexual dimorphism
of karyotype, and inheritance analysis of sex-linked
genes. The first method is not easily applied to branchiopods because of their relatively short life cycle
and rather delicate constitution but has been useful
in investigating other groups of animals (Humphrey,
1945; Katakura, 1984). The second has been applied
to some Anostraca (Bianchi Bullini et aI., 1968), leading to the suggestion that in Artemia females are heterogametic whereas in some other forms the males are
hemigametic. Chromosomes of Conchostraca are quite
small (Zaffagnini, 1969), and methods have yet to be
developed for the reliable analysis of their karyotypes.
I have found the third method, however, to be quite
useful in the analysis of sex determining mechanisms
of clam shrimps, although it has to date been applied
only to Eocyzicus concavus.
The joint inheritance of offspring sex and genotype
at an enzyme locus with electrophoretically detectable
alleles in E. concavus demonstrates female heterogamety (Sassaman, 1990; unpubl. obs.). The locus (Idh2, encoding the enzyme NADP-dependent isocitrate
dehydrogenase) is in double-copy in both sexes, but
the inheritance of alleles is partially sex-linked. Backcrosses of heterozygous males yield progeny genotype
distributions that are random with respect to offspring
sex; however, backcrosses of heterozygous females
yield a strong (nearly perfect) correlation between offspring sex and offspring genotype. This pattern of
inheritance is phase-dependent; the specific association between offspring sex and genotype can be manipulated by changing the origin of the alleles in the tested
female. The overall pattern of inheritance is consistent with female heterogamety and tight linkage of the
marker locus (Idh-2) with the region of the genome
carrying sex determining elements. (Limited data also
indicate tight linkage of another enzyme-encoding
locus - Fum, fumarate dehydratase - to both the sex
determining locus and Idh-2.) This pattern of partially sex-linked inheritance of enzyme-encoding genes
in Eocyzicus is similar to that previously described for