Evolution of Other Features
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We have used our preliminary estimate of Rhabditidae phylogeny (Tree 2) to trace the evolution of some interesting features of Rhabditidae morphology and biology:

 

Evolution of sexual systems

  • In Rhabditidae, hermaphroditism (self-fertilization) evolved several times independently from gonochoristic (male-female) lineages (see figure).
  • On the other hand, there is no convincing evidence for the converse.
  • These data are consistent with the hypothesis that hermaphroditism offers short-term selective advantages under some conditions, but that such lineages may have a higher extinction rate.

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Evolution of the "glottoid apparatus"

  • The glottoid apparatus is a small swelling at the base of the buccal tube in many Rhabditidae, and distinguishes Rhabditidae from other species.
  • The absence of this structure was classically deemed "primitive"; species groups without a glottoid apparatus were assumed to have diverged ancestrally (e.g., Protorhabditis, Parasitorhabditis).
  • Out data show that the glottoid apparatus has been lost at least twice in Rhabditidae, and that these species groups are recently derived (see figure).

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Heterotopy of the vulva correlates with gonad morphology

  • In nematodes, heterotopy (evolutionary change in position) of the vulva occurs.  For example, the vulva can be midbody or posterior.
  • Female gonads can have 1 or 2 "arms" (mono- or didelphy), and the uteri can point anteriorly (prodelphy), posteriorly (opisthodelphic), or in opposite directions (amphidelphic).
  • Our phylogenetic analyses show a significant correlation between a posterior vulva and loss of the posterior gonadal arm (prodelphic monodelphy) (see figure).

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Webmeister: David Fitch (C) 1999 NYUtorch.gif
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