54
white-eyed mutants in two species of Artemia (Bowen,
1963; 1965; Barigozzi et al., 1969).
The genetical attributes of obligately sexual conchostracans are incorporated into a diagram of the life
cycle in Fig. 2A. The essential features are that sexes
are genetically specified, females are heterozygous for
the sex-determining factor, and all offspring are produced by outcrossing. In obligately sexual life cycles,
the inference that females are heterozygous is logically
equivalent to the inference that the male-determining
genetic factor is recessive in its expression. Thus,
females have a genotypic constitution of Sis at the
sex-determining locus whereas males are sis.
Androdioecy
In species characterized by female-biased sex ratios,
sex ratios often vary spatially in natural populations
(Table 1) and the greater the female-bias, the greater
is the degree of inbreeding as measured by genotype
frequencies at loci with codominant (electrophoretically detectable) polymorphisms (Sassaman, 1989).
Furthermore, in laboratory experiments, mating is not
required (Belk, 1972; Strenth & Sissom, 1975) and
individual clutch sex ratios are usually not 1 male: 1
female (Sassaman, 1988; Sassaman & Weeks, 1993).
In both Eulimnadia texana and E. antlei there is a
polymorphism involving two categories of females:
monogenics that produce only female offspring and
amphigenics that produce a clutch containing males
and females in a 1:3 ratio. The outcrossed offspring
of monogenics are themselves exclusively amphigenics and the outcrossed offspring of amphigenics are
males and females in a 1: 1 ratio (Sassaman & Weeks,
1993).
These two phenotypes represent two alternative
genotypes, one heterozygous for a sex-determining
element (Sis) and one homozygous for the 'femaledetermining' genetic element (SIS) (Sassaman &
Weeks, 1993). As in Eocyzicus concavus, the Idh2 and Fum genes are linked to the factor involved
in sex-determination. The common synteny of Idh-2,
Fum, and the sex-determining factor in Eulimnadia and
Eocyzicus suggests homology of the sex-determining
genes. Given the antiquity of the divergence of Limnadiidae from Cyzicidae (Wright, 1920; Zhang et al.,
1990), this relationship further indicates a remarkable
conservation of gene arrangement during the last several hundred million years.
A further inference can be made regarding the
nature of the genetic control of sex determination
in Eulimnadia. The two genotypes of females are
phenotypically indistinguishable from each other by
any superficial characteristics; they can only be differentiated on the basis of the clutch sex ratios that
they produce. This phenotypic similarity suggests that
the genotypic differences between the two alternative
types of females is due to a relatively small number of genes rather than to a large chromosomal difference. Otherwise, heterozygous and homozygous
females would differ at a large number of loci, and any
polymorph isms at those loci would be expressed differently in the two female forms (Sassaman & Weeks,
1993).
The comparison between these two genera suggests
that the genetic system in Eulimnadia is derived from
a system like that in Eocyzicus; the basic difference
being that in Eulimnadia females are capable of reproducing either by sexual outcrossing or by facultative
selfing (Sassaman & Weeks, 1993). As a consequence
of selfing, a second female genotype is expressed. The
life cycle thus consists of two female genotypes each
capable of two alternative modes of reproduction, as
illustrated in Fig. 2B. This model of the androdioecious life cycle has been extensively tested by formal
pedigree analysis in Eulimnadia texana (Sassaman &
Weeks, 1993), and elements of it have also been established in E. antlei (Sassaman, 1988).
Un isexuality
Although there has been no inheritance analysis of
the genetics of sex-determination in unisexual species,
certain genetic inferences can be made on the basis
of our understanding from sexual and androdioecious
life cycles that the male-determining factor is recessive in expression. If segregation of alleles occurs during female meiosis, then females must be homozygous for female-determining factors; otherwise males
would be expressed in the next generation (Fig. 2E).
In contrast, if segregation does not occur, but females
transmit their own genotype to their daughters intact,
then females can either be heterozygous for the sexdetermining locus (Fig. 2C) or they can be homozygous
(Fig.2D).
These three cases are not easily distinguished from
each other. Inheritance patterns of codominant alleles (such as in enzyme polymorphisms detectable by
protein electrophoresis) could distinguish selfing from
parthenogenetic reproduction. If females reproduced
by selfing, then offspring from a heterozygous female
would be a binomial mixture of heterozygotes and the
white-eyed mutants in two species of Artemia (Bowen,
1963; 1965; Barigozzi et al., 1969).
The genetical attributes of obligately sexual conchostracans are incorporated into a diagram of the life
cycle in Fig. 2A. The essential features are that sexes
are genetically specified, females are heterozygous for
the sex-determining factor, and all offspring are produced by outcrossing. In obligately sexual life cycles,
the inference that females are heterozygous is logically
equivalent to the inference that the male-determining
genetic factor is recessive in its expression. Thus,
females have a genotypic constitution of Sis at the
sex-determining locus whereas males are sis.
Androdioecy
In species characterized by female-biased sex ratios,
sex ratios often vary spatially in natural populations
(Table 1) and the greater the female-bias, the greater
is the degree of inbreeding as measured by genotype
frequencies at loci with codominant (electrophoretically detectable) polymorphisms (Sassaman, 1989).
Furthermore, in laboratory experiments, mating is not
required (Belk, 1972; Strenth & Sissom, 1975) and
individual clutch sex ratios are usually not 1 male: 1
female (Sassaman, 1988; Sassaman & Weeks, 1993).
In both Eulimnadia texana and E. antlei there is a
polymorphism involving two categories of females:
monogenics that produce only female offspring and
amphigenics that produce a clutch containing males
and females in a 1:3 ratio. The outcrossed offspring
of monogenics are themselves exclusively amphigenics and the outcrossed offspring of amphigenics are
males and females in a 1: 1 ratio (Sassaman & Weeks,
1993).
These two phenotypes represent two alternative
genotypes, one heterozygous for a sex-determining
element (Sis) and one homozygous for the 'femaledetermining' genetic element (SIS) (Sassaman &
Weeks, 1993). As in Eocyzicus concavus, the Idh2 and Fum genes are linked to the factor involved
in sex-determination. The common synteny of Idh-2,
Fum, and the sex-determining factor in Eulimnadia and
Eocyzicus suggests homology of the sex-determining
genes. Given the antiquity of the divergence of Limnadiidae from Cyzicidae (Wright, 1920; Zhang et al.,
1990), this relationship further indicates a remarkable
conservation of gene arrangement during the last several hundred million years.
A further inference can be made regarding the
nature of the genetic control of sex determination
in Eulimnadia. The two genotypes of females are
phenotypically indistinguishable from each other by
any superficial characteristics; they can only be differentiated on the basis of the clutch sex ratios that
they produce. This phenotypic similarity suggests that
the genotypic differences between the two alternative
types of females is due to a relatively small number of genes rather than to a large chromosomal difference. Otherwise, heterozygous and homozygous
females would differ at a large number of loci, and any
polymorph isms at those loci would be expressed differently in the two female forms (Sassaman & Weeks,
1993).
The comparison between these two genera suggests
that the genetic system in Eulimnadia is derived from
a system like that in Eocyzicus; the basic difference
being that in Eulimnadia females are capable of reproducing either by sexual outcrossing or by facultative
selfing (Sassaman & Weeks, 1993). As a consequence
of selfing, a second female genotype is expressed. The
life cycle thus consists of two female genotypes each
capable of two alternative modes of reproduction, as
illustrated in Fig. 2B. This model of the androdioecious life cycle has been extensively tested by formal
pedigree analysis in Eulimnadia texana (Sassaman &
Weeks, 1993), and elements of it have also been established in E. antlei (Sassaman, 1988).
Un isexuality
Although there has been no inheritance analysis of
the genetics of sex-determination in unisexual species,
certain genetic inferences can be made on the basis
of our understanding from sexual and androdioecious
life cycles that the male-determining factor is recessive in expression. If segregation of alleles occurs during female meiosis, then females must be homozygous for female-determining factors; otherwise males
would be expressed in the next generation (Fig. 2E).
In contrast, if segregation does not occur, but females
transmit their own genotype to their daughters intact,
then females can either be heterozygous for the sexdetermining locus (Fig. 2C) or they can be homozygous
(Fig.2D).
These three cases are not easily distinguished from
each other. Inheritance patterns of codominant alleles (such as in enzyme polymorphisms detectable by
protein electrophoresis) could distinguish selfing from
parthenogenetic reproduction. If females reproduced
by selfing, then offspring from a heterozygous female
would be a binomial mixture of heterozygotes and the