Hydrobiologia 298: 83-86, 1995.
D. Belle, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
83
Selfing versus outcrossing in the androdioecious clam shrimp,
Eulimnadia texana (Crustacea, Conchostraca)
Lana Knoll & Naida Zucker
Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA
Key words: androdioecious crustacean, Eulimnadia texana, outcrossing, selfing, clutch size
Abstract
The clam shrimp Eulimnadia texana is an androdioecious crustacean in which hermaphrodites may self fertilize
or outcross with males but cannot outcross with other hermaphrodites. Outcrossing is maintained within most
popUlations of this species despite the high genetic cost of sex, suggesting that compensating factors provide
an advantage to outcrossing. We hypothesized that one such benefit would be the production of larger clutch
sizes resulting from outcrossed matings. To test this prediction, we recorded the body sizes and clutch sizes of
hermaphrodites which mated via selfing or via outcrossing. Clutch sizes showed significant, almost exponential,
increases as body size increased in both selfing and outcrossing hermaphrodites. The rate of this increase was the
same for both groups, and there was no significant difference in clutch size when body size was controlled for
between the two fertilization types.
Introduction
Eulimnadia texana exhibits androdioecy, a rare condition in animal mating systems in which both
hermaphrodites and males are present within a species
(Sassaman & Weeks, 1993). Such a system provides an
excellent framework for the study of the significance
of outcrossing versus selfing. Hermaphrodites can fertilize their own eggs or outcross with males but are
unable to exchange sperm with other hermaphrodites.
The male's first two pairs of thoracic appendages
have been modified into claspers so that he can
grasp the hermaphrodite during mating. The lack of
these claspers physically prevents the hermaphrodites
from mating with one another (Sassaman & Weeks,
1993).
A hermaphrodite that selfs its eggs potentially
produces as many progeny as those produced by a
malelhermaphrodite pair, but these progeny possess
twice as many of the hermaphrodite's genes. Theoretically, it would be advantageous for a hermaphrodite
to self all of its offspring, assuming there are no
counteracting factors such as inbreeding depression
(Charlesworth, 1980; Otto et al., 1993; Sassaman
& Weeks, 1993). Eulimnadia texana individuals are
either monogenic hermaphrodites (those which produce only hermaphroditic offspring when selfing),
amphigenic hermaphrodites (those which produce both
male and hermaphroditic offspring when selfing), or
males (Sassaman & Weeks, 1993). Monogenic individuals are homozygous dominant at the sex-determining
autosomal locus, amphigenic individuals are heterozygous, and males are homozygous recessive (Sassaman
& Weeks, 1993). Due to the nature of sex determination within this system, populations characteristically
have many more hermaphrodites than males and continual selfing of progeny could lead to the eventual
loss of amphigenic hermaphrodites and males within populations (Sassaman & Weeks, 1993). Selfing in
E. texana is common and selfing occurs even when
males are present (Sassaman, 1989; Knoll & Zucker,
1995). There are populations in which there are no
males and presumably no amphigenic hermaphrodites
(Sassaman, 1989; Knoll, 1995). Studies have suggested that males are less viable than hermaphrodites under
both field and laboratory conditions (Strenth, 1977;
Sassaman & Weeks, 1993; Knoll, 1995). Low male
viability may also contribute to higher levels of selfing in E. texana. Other populations of E. texana have
continued to maintain relatively high percentages of
D. Belle, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
83
Selfing versus outcrossing in the androdioecious clam shrimp,
Eulimnadia texana (Crustacea, Conchostraca)
Lana Knoll & Naida Zucker
Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA
Key words: androdioecious crustacean, Eulimnadia texana, outcrossing, selfing, clutch size
Abstract
The clam shrimp Eulimnadia texana is an androdioecious crustacean in which hermaphrodites may self fertilize
or outcross with males but cannot outcross with other hermaphrodites. Outcrossing is maintained within most
popUlations of this species despite the high genetic cost of sex, suggesting that compensating factors provide
an advantage to outcrossing. We hypothesized that one such benefit would be the production of larger clutch
sizes resulting from outcrossed matings. To test this prediction, we recorded the body sizes and clutch sizes of
hermaphrodites which mated via selfing or via outcrossing. Clutch sizes showed significant, almost exponential,
increases as body size increased in both selfing and outcrossing hermaphrodites. The rate of this increase was the
same for both groups, and there was no significant difference in clutch size when body size was controlled for
between the two fertilization types.
Introduction
Eulimnadia texana exhibits androdioecy, a rare condition in animal mating systems in which both
hermaphrodites and males are present within a species
(Sassaman & Weeks, 1993). Such a system provides an
excellent framework for the study of the significance
of outcrossing versus selfing. Hermaphrodites can fertilize their own eggs or outcross with males but are
unable to exchange sperm with other hermaphrodites.
The male's first two pairs of thoracic appendages
have been modified into claspers so that he can
grasp the hermaphrodite during mating. The lack of
these claspers physically prevents the hermaphrodites
from mating with one another (Sassaman & Weeks,
1993).
A hermaphrodite that selfs its eggs potentially
produces as many progeny as those produced by a
malelhermaphrodite pair, but these progeny possess
twice as many of the hermaphrodite's genes. Theoretically, it would be advantageous for a hermaphrodite
to self all of its offspring, assuming there are no
counteracting factors such as inbreeding depression
(Charlesworth, 1980; Otto et al., 1993; Sassaman
& Weeks, 1993). Eulimnadia texana individuals are
either monogenic hermaphrodites (those which produce only hermaphroditic offspring when selfing),
amphigenic hermaphrodites (those which produce both
male and hermaphroditic offspring when selfing), or
males (Sassaman & Weeks, 1993). Monogenic individuals are homozygous dominant at the sex-determining
autosomal locus, amphigenic individuals are heterozygous, and males are homozygous recessive (Sassaman
& Weeks, 1993). Due to the nature of sex determination within this system, populations characteristically
have many more hermaphrodites than males and continual selfing of progeny could lead to the eventual
loss of amphigenic hermaphrodites and males within populations (Sassaman & Weeks, 1993). Selfing in
E. texana is common and selfing occurs even when
males are present (Sassaman, 1989; Knoll & Zucker,
1995). There are populations in which there are no
males and presumably no amphigenic hermaphrodites
(Sassaman, 1989; Knoll, 1995). Studies have suggested that males are less viable than hermaphrodites under
both field and laboratory conditions (Strenth, 1977;
Sassaman & Weeks, 1993; Knoll, 1995). Low male
viability may also contribute to higher levels of selfing in E. texana. Other populations of E. texana have
continued to maintain relatively high percentages of