Parthenogenetic Mechanism and Its Evolutionary Potential in the Cave Cricket Euhadenoecus insolitus (Orthoptera: Rhaphidophoridae)

Abstract

Populations of the North American cave cricket, Euhadenoecus insolitus Hubbell, are either parthenogenetic or bisexual, and both types are diploid. The female-only populations tend to be on the northern periphery of the normal range of the species. By microdensitometry, the parthenogenetic mechanism was shown to allow the haploid egg nuclei to multiply to the 256-nuclei (blastoderm) stage, at which point many haploid nuclei became diploid. The percentage of eggs developing to blastoderm stage from parthenogenetic populations was as much as 13 times that of unfertilized eggs from bisexual populations and was equivalent to that of fertilized eggs. Restoration of diploidy at an advanced stage of embryonic development is rare in insects, previously reported only in two species of Phasmatodea. Females and eggs from bisexual and parthenogenetic populations are indistinguishable morphologically, including structures that are necessary for normal fertilization (the adult spermatheca and egg micropyles). In the laboratory, a mating between a male and a parthenote has been achieved, and the sperm were stored alive in her spermatheca. It is postulated that female-only populations arise as founding events by parthenogenetically quasi-competent virgin females from bisexual colonies, and that some of these parthenogenetic colonies may become secondarily bisexual with the later advent of males.