Evolution Of Reproductive Mode

Of the two major divisions of the cockroaches, the super-families Blattoidea and Blaberoidea (McKittrick, 1964), most evolutionary drama with regard to reproductive mode is in the latter. It includes the Blattellidae, in which some species retain the egg case externally for the entire period of gestation, and where ovoviviparity arose independently in two different subfamilies. It also includes the Blaberidae, all of which incubate egg cases internally, suggesting that they have radiated since an ancestor acquired the trait. The sole viviparous genus, as well as the group that lost the oothecal covering, are in the Blaberidae. Of course, critical analysis of the pattern of reproductive evolution is dependent on the availability of robust phy-logenies for the groups under study, but, as with most aspects of cockroach systematics, the relationships among several subgroups of the Blaberoidea are unsettled. In all phylogenetic hypotheses proposed so far, however, Bla-beridae is most closely related to Blattellidae (Roth, 2003c), and some studies (Klass, 1997,2001) suggest that blaberids are a subgroup of the Blattellidae.

The evolution of reproductive mode in cockroaches can be described with some confidence as a unidirectional trend from oviparity to viviparity, without character reversals. Reproduction is an extraordinarily complex process, with morphology, physiology, and behavior integrated and coordinated by neural and endocrine mechanisms. Transitions therefore tend to be irreversible due to genetic or physiological architecture, or because strong selection on offspring prevents them (Tinkle and Gibbons, 1977;Crespi and Semeniuk, 2004).An initial step in the evolution of ovoviviparity in cockroaches was likely to be facultative transport of the egg case, as in the oviparous type A species that retain oothecae until a suitable microhabitat is found. Ectobiuspallidus, for example, typically deposits its egg case in one or two days, but has been reported to carry it 16 days or longer (Roth and Willis, 1958a). Therea petiveriana deposits the ootheca within a day of extrusion, but may retain it for as long as 90 hr if a suitably moist substrate is not available (Livingstone and Ramani, 1978). From this flexible starting point, the trend toward ovoviviparity would be exemplified by cockroaches that retain the egg case for the entire period of embryogenesis, but provide no materials additional to those originally in the egg case. Currently, there are no records of extant cockroaches that exhibit this pattern; the only oviparous type B species that has been studied, B. germanica, provides water and soluble materials to embryos. Obligate egg retention evolves when maternal tissues became responsive to the attached egg case; this recognition then induces further modifications of maternal function (Guillette, 1989).

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