56
Cyclic parthenogenesis
The reproductive biology of Cyclestheria hislopi has
attracted considerable attention since Sars' original
description (1887) of the first, laboratory-reared, male.
Nevertheless, detailed knowledge on this species is
lacking. Unlike most other conchostracans, Cyclestheria has the capacity for two distinct modes of egg
production: one mode leads to direct-developing (subitaneous) eggs that are brooded by the female until
juveniles hatch (Sars, 1887; Paul & Nayar, 1977;
Roessler & Sanchez, 1986; Roessler, 1994c) and the
other mode leads to 'resting' or diapause eggs that are
enveloped in a protective shield formed from the carapace of the female and resembling a primitive ephippium (Roessler & Sanchez, 1986; Roessler, 1995c).
Only females hatch from diapause eggs (Sars, 1887;
Roessler & Sanchez, 1986; Roessler, 1995c), but
direct-developing eggs can hatch into either all-female
clutches (Paul & Nayar, 1977) or mixed-sex clutches
(Roessler & Sanchez, 1986; Roessler, 1995c), apparently in response to environmental stimuli.
The species occurs in both lotic habitats (Michael,
1968; Petr, 1968; Junk, 1977; Egborge& Ozoro, 1989;
Poi de Nieff& Bruquetas de Zozaya, 1991) and lentic
habitats (Sars, 1887; Botnariuc & ViTia Bayes, 1977;
Paul & Nayar, 1977; Sissom, 1980; Timms, 1986;
Roessler & Sanchez, 1986; Roessler, 1995c). The
lentic habitats occupied by Cyclestheria may be temporary or they may be perennially filled with water,
but varying seasonally in environmental conditions.
The occurrence of males differs among the habitat
types. Only females are found in lotic environments,
and temporal sampling indicates that there are iterative generations of unisexuality (Egborge & Ozoro,
1989). In lentic habitats there are initial generations of
females, but often, as the pools contract, subsequent
generations include some males (Roessler & Sanchez,
1986; Roessler, 1995c). By analogy with certain cladocerans, whose reproductive biology is better known,
the reproductive cycle of Cyclestheria is called 'cyclic
parthenogenesis' to convey the interpretation that the
normal cycle is a series of iterative generations of unisexual reproduction punctuated by the expression of
sexually differentiated individuals whose matings produce the 'resting eggs' necessary in temporary environments. In this model, the cycle in lotic habitats is
uninterrupted by environmental decline, and only the
iterative unisexual reproduction is expressed.
The critical, but as yet unresolved, issue in understanding the cyclestherian life cycle is the nature of
sex determination. Do environmental stimuli directly cause some embryos to develop into males, or do
they simply trigger a change in the female's reproductive processes that causes the phenotypic expression
of genetically specified males? If the primitive, genetically based, mechanism of sex determination in other
conchostracans has been supplanted by environmental
sex determination in cyclestherians, then these latter
life cycles might be appropriately inferred using the
analogy of cladocerans (Hebert, 1987; Hobrek & Larsson, 1990). The alternative viewpoint, which I will
develop here, is that cyclestherian life cycles can be
envisioned as modifications of the same genetically
based life cycles operating in other conchostracans.
If the sexual males and females induced in later generations have a genetic basis comparable to the genetics
of gender in other Conchostraca, then females of the
earlier generations must carry both male-determining
and female-determining factors. Thus they may be
viewed formally as heterozygous (Sis) for the sexdetermining locus. For their iterative reproduction to
occur without the expression of males, they must reproduce parthenogenetically. The terminal generations in
which males are expressed then represent a modified
form of egg production in which the segregation of
the male-determining factors occurs. This biphasic life
cycle is diagrammed in Fig. 2F, which illustrates two
distinct cycles of reproduction, one parthenogenetic
and one in which sexual expression of the underlying genetic system is induced by environmental stimuli.
The evolution of unisexuality
The phylogenetic analysis of population sex ratio, calibrated by laboratory studies on selected species, indicates that the ancestral condition of conchostracans
was one of obligately sexual reproduction (Fig. 1).
The similarity in the genetical attributes of sex determination between Eulimnadia texana and Eocyzicus
concavus further suggests that the ancestral condition
involved female heterozygosity (male recessiveness)
for a single gender-specifying factor. These attributes
are taken to represent the ancestral condition of sex
determination and mode of reproduction in conchostracans (Fig. 2A).
The current conditions in female-biased and unisexual lineages have been described in the preceding section and are summarized diagrammatically in
Fig. 2B-F. What remains to be done is to provide sce-
Cyclic parthenogenesis
The reproductive biology of Cyclestheria hislopi has
attracted considerable attention since Sars' original
description (1887) of the first, laboratory-reared, male.
Nevertheless, detailed knowledge on this species is
lacking. Unlike most other conchostracans, Cyclestheria has the capacity for two distinct modes of egg
production: one mode leads to direct-developing (subitaneous) eggs that are brooded by the female until
juveniles hatch (Sars, 1887; Paul & Nayar, 1977;
Roessler & Sanchez, 1986; Roessler, 1994c) and the
other mode leads to 'resting' or diapause eggs that are
enveloped in a protective shield formed from the carapace of the female and resembling a primitive ephippium (Roessler & Sanchez, 1986; Roessler, 1995c).
Only females hatch from diapause eggs (Sars, 1887;
Roessler & Sanchez, 1986; Roessler, 1995c), but
direct-developing eggs can hatch into either all-female
clutches (Paul & Nayar, 1977) or mixed-sex clutches
(Roessler & Sanchez, 1986; Roessler, 1995c), apparently in response to environmental stimuli.
The species occurs in both lotic habitats (Michael,
1968; Petr, 1968; Junk, 1977; Egborge& Ozoro, 1989;
Poi de Nieff& Bruquetas de Zozaya, 1991) and lentic
habitats (Sars, 1887; Botnariuc & ViTia Bayes, 1977;
Paul & Nayar, 1977; Sissom, 1980; Timms, 1986;
Roessler & Sanchez, 1986; Roessler, 1995c). The
lentic habitats occupied by Cyclestheria may be temporary or they may be perennially filled with water,
but varying seasonally in environmental conditions.
The occurrence of males differs among the habitat
types. Only females are found in lotic environments,
and temporal sampling indicates that there are iterative generations of unisexuality (Egborge & Ozoro,
1989). In lentic habitats there are initial generations of
females, but often, as the pools contract, subsequent
generations include some males (Roessler & Sanchez,
1986; Roessler, 1995c). By analogy with certain cladocerans, whose reproductive biology is better known,
the reproductive cycle of Cyclestheria is called 'cyclic
parthenogenesis' to convey the interpretation that the
normal cycle is a series of iterative generations of unisexual reproduction punctuated by the expression of
sexually differentiated individuals whose matings produce the 'resting eggs' necessary in temporary environments. In this model, the cycle in lotic habitats is
uninterrupted by environmental decline, and only the
iterative unisexual reproduction is expressed.
The critical, but as yet unresolved, issue in understanding the cyclestherian life cycle is the nature of
sex determination. Do environmental stimuli directly cause some embryos to develop into males, or do
they simply trigger a change in the female's reproductive processes that causes the phenotypic expression
of genetically specified males? If the primitive, genetically based, mechanism of sex determination in other
conchostracans has been supplanted by environmental
sex determination in cyclestherians, then these latter
life cycles might be appropriately inferred using the
analogy of cladocerans (Hebert, 1987; Hobrek & Larsson, 1990). The alternative viewpoint, which I will
develop here, is that cyclestherian life cycles can be
envisioned as modifications of the same genetically
based life cycles operating in other conchostracans.
If the sexual males and females induced in later generations have a genetic basis comparable to the genetics
of gender in other Conchostraca, then females of the
earlier generations must carry both male-determining
and female-determining factors. Thus they may be
viewed formally as heterozygous (Sis) for the sexdetermining locus. For their iterative reproduction to
occur without the expression of males, they must reproduce parthenogenetically. The terminal generations in
which males are expressed then represent a modified
form of egg production in which the segregation of
the male-determining factors occurs. This biphasic life
cycle is diagrammed in Fig. 2F, which illustrates two
distinct cycles of reproduction, one parthenogenetic
and one in which sexual expression of the underlying genetic system is induced by environmental stimuli.
The evolution of unisexuality
The phylogenetic analysis of population sex ratio, calibrated by laboratory studies on selected species, indicates that the ancestral condition of conchostracans
was one of obligately sexual reproduction (Fig. 1).
The similarity in the genetical attributes of sex determination between Eulimnadia texana and Eocyzicus
concavus further suggests that the ancestral condition
involved female heterozygosity (male recessiveness)
for a single gender-specifying factor. These attributes
are taken to represent the ancestral condition of sex
determination and mode of reproduction in conchostracans (Fig. 2A).
The current conditions in female-biased and unisexual lineages have been described in the preceding section and are summarized diagrammatically in
Fig. 2B-F. What remains to be done is to provide sce-