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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section IVOther Directions

Baird et al. (1992) have extended the early investigations by Osche (1952) into the mechanisms of species isolation among the most closely related Caenorhabditis species. Using Caenorhabditis for such studies obviously takes advantage of C. elegans genetics to understand the genetic basis of reproductive isolation. One result has been the reaffirmation of “Haldane's Rule” (hybrid inviability affects the heterogametic sex); further genetic tests can address predictions made from other systems (see, e.g., Orr 1993).

Hermaphroditism has evolved several times in the history of the Rhabditidae (Maupas 1900; Potts 1910; Sudhaus 1976), but the evolutionary mechanism for why this reproductive strategy has arisen poses interesting problems at both the genetic level and the population level. Sperm-limited fecundity, as seen in these hermaphrodites, is unusual in Metazoa. Using C. elegans mutants in which the timing of the spermatogenesis-to-oogenesis switch varied, Hodgkin and Barnes (1991) showed that the timing of this switch is critical in maximizing population growth rate on a limited resource. The population dynamics have also been modeled (Barker 1992). One model has been proposed that derives an optimum self-fertile hermaphrodite/male sex ratio and predicts that the C. elegans mating system will select for males with promiscuous copulatory behavior and largely “disinterested” hermaphrodites (Hedgecock 1976). The evolution of sex determination itself poses an enormously interesting problem as well. Although sex determination in flies, worms, mice, and humans involves a clear binary fate decision that is genetically determined, the overall mechanisms are not conserved (Parkhurst and Meneely 1994), and it is presently unclear how this diversity has evolved.

Ecological and coevolutionary studies of interactions between nematodes and other species have provided classic examples of ecological succession (see, e.g., Sudhaus 1981), host-parasite specificity (see, e.g., Mitter and Brooks 1983; Blaxter and Bird, this volume), the origins of parasitism (see, e.g., Sudhaus and Schulte 1988), and so on; but current studies have also focused on interactions involving Caenorhabditis species (see, e.g., Sudhaus 1974; Sudhaus and Kühne 1989; Baird et al. 1994). For example, K. Kiontke (pers. comm.) has shown a highly specific phoretic relationship between a Caenorhabditis species and a drosophilid fruit fly. Although identification of the loci involved in determining this interaction is a long way off, it is intriguing to speculate that two genetic model systems could be used to study the evolution of an ecological relationship at the genetic level.

Obviously, many other fundamental parameters of nematode evolution have yet to be explored (with or without C. elegans as a model), such as dispersal and gene flow between natural populations, ecological niches and species interactions, and life history strategies.

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20154
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