Seattle Central Community College

Science & Math Department

1701 Broadway

Seattle, WA USA
 
 

hodin@alumni.washington.edu
 
 
 
 

"The ecdysone receptor and the parallel evolution of paedogenesis (larval reproduction) in two species of gall midges (Diptera: Cecidomyiidae)"  (abstract number 1515)
 

Although most insects reproduce in the adult stage, facultative larval or pupal reproduction (paedogenesis) has evolved at least six times indepently in insects, twice in gall midges of the family Cecidomyiidae (Diptera). Paedogenesis in cecids involves the precocious growth and differentiation of the ovary in an otherwise larval form. We have previously shown (Hodin & Riddiford, 1998) that the timing of expression of the Ecdysone Receptor (EcR) and Ultraspiracle (USP), the two proteins that constitute the functional receptor for the steroid hormone 20-hydroxyecdysone, regulates the timing and progression of ovarian differentiation in Drosophila melanogaster (Diptera: Drosophilidae). Here I test the hypothesis that precocious activation of EcR and USP in the ovaries of paedogenetic gall midges allows for precocious ovarian differentiation. Using monoclonal antibodies directed against insect EcR and USP proteins, I first show that when these gall midges are reared under conditions that promote typical, metamorphic development, up-regulation of EcR and USP occurs in the final larval stage. By contrast, in the paedogenetic life cycle, EcR and USP are up-regulated early in the first larval stage. A similar pattern is seen for two independently-evolved paedogenetic gall midges, Heteropeza pygmaea and Mycophila speyeri.  Thus, I conclude that the mechanisms underlying precocious growth of the ovary evolved in parallel in these two lineages.

Of the six independent instances of paedogenesis (defined broadly to include pupal reproduction, as seen in some chironomid midges), four of them are among the Diptera.  I argue that the paucity of instances of paedogenesis outside of the "lower" Diptera represents a developmental constraint on the evolution of paedogenesis.  I will propose that certain ontogenetic features unique to the lower Diptera predispose them to evolve a paedogenetic life cycle.
 
 
 
 

PERSONAL STATEMENT

I'm interested in evolutionary patterns, and how those patterns may be generated by constraints or other biases in ontogenetic mechanisms. I look for developmentally-tractable organisms that: 1) exhibit variability in some developmentally interesting character or characters; 2) come from a group with some ideas as to their phylogenetic relationships; and 3) show some evolutionary pattern (such as evolutionary novelty or homoplasy) for that (those) character(s). From such a starting point, I try to determine the developmental underpinnings of the evolutionary pattern. In this context, I have studied the developmental basis of plasticity (environmentally-based variability within a genotype) and inter-species variation (genetically-fixed variability) for ovariole number (a reproductive character) in the fruit fly Drosophila melanogaster and its close relatives. I have undertaken a similar study of worker reproduction in the Cape honeybee (Apis mellifera capensis). At this symposium, I will discuss some work on parallel evolution of larval reproduction (paedogenesis) in the Cecidomyiidae (the "gall midge" family of flies). I feel that by uncovering the ontogenetic bases for differences among closely related species, we can begin to build a theoretical frame-work for the nascent field of Evo-Devo.
 
 

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