SOME EVOLUTIONARY POSSIBILITIES FOR A MICROBE THAT CAUSES INCOMPATIBILITY IN ITS HOST.

Abstract

This note concerns some speculations about the evolution of Wolbachia, an intracellular bacterium found in many insects. O'Neill et al. (1992) reported the results of a survey of a limited number of insects, and more recently R. Giordano and H. M. Robertson have found it in more than 10% of more than 500 species surveyed (pers. comm. 1992). In several insects, it has been shown to cause unidirectional incompatibility between males carrying the microbe and females that do not (O'Neill et al. 1992; Stevens and Wade 1990). In haplo-diploids, it can cause sex-ratio distortion and parthenogenesis (Stouthammer et al. 1993). This note concerns the incompatibility effect, in which the Wolbachia spreads by partially sterilizing those females that do not carry it. Because of this mode of increase, Hurst et al. (1992) referred to the organism as a selfish gene. Rousset and Raymond (1991) titled their paper on the subject Why sterilize females? and compared the system to spiteful behavior. One objective of this note is to illustrate how such anthropomorphic metaphors might be misleading and cause diversion from interesting and perhaps useful questions. The exercise that follows examines the population behavior of possible kinds of nonsterilizing strains of Wolbachia. This system, then, has three kinds of hosts: unifected hosts, u; hosts carrying Wolbachia with the normal sterilizing allele, s; and hosts carrying the microbe with one or the other of two kinds of nonsterilizing alleles, explained below. The recurrence equations for this system were derived in an analogous way to that of Nigro and Prout (1990) who also had three host states. The assumptions are random mating of the host, matriclinous transmission of the microbe, and discrete generations. Additional biology is identified in the parameter definitions below. The equations are