The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp.
The nematode worm Caenorhabditis elegans and the clam shrimp Eulimnadia texana are two well-studied androdioecious species consisting mostly of self-fertilizing hermaphrodites and few males. To understand how androdioecy can evolve, a simple two-step mathematical model of the evolutionary pathway from a male-female species to a selfing-hermaphrodite species is constructed. First, the frequency of mutant females capable of facultative self-fertilization increases if the benefits of reproductive assurance exceed the cost. Second, hermaphrodites become obligate self-fertilizers if the fitness of selfed offspring exceeds one-half the fitness of outcrossed offspring. Genetic considerations specific to C. elegans and E. texana show that males may endure as descendants of the ancestral male-female species. These models combined with an extensive literature review suggest a sexual conflict over mating in these androdioecious species: selection favours hermaphrodites that self and males that outcross. The strength of selection on hermaphrodites and males differs, however. Males that fail to outcross suffer a genetic death. Hermaphrodites may never encounter a rare male, and those that do and outcross only bear less fecund offspring. This asymmetric sexual conflict results in an evolutionary stand-off: rare, but persistent males occasionally fertilize common, but reluctant hermaphrodites. A consequence of this stand-off may be an increase in the longevity of the androdioecious mating system.
Chasnov
J R
JR
Department of Mathematics, Hong Kong University of Science and Technology, Kowloon, Hong Kong. jeffrey.chasnov@ust.hk
D000818:Animals; D005075:Biological Evolution; D017173:Caenorhabditis elegans / Q000235:genetics*; D003445:Crustacea / Q000235:genetics*; D012734:Disorders of Sex Development / Q000235:genetics; D005260:Female; D007178:Inbreeding; D008297:Male; D008957:Models, Genetic; D012641:Selection, Genetic