Incidence of Male-Killing Rickettsia spp. (α-Proteobacteria) in the Ten-Spot Ladybird Beetle Adalia decempunctata L. (Coleoptera: Coccinellidae)

The diversity of endosymbiotic bacteria that kill male host offspring during embryogenesis and their frequencies in certain groups of host taxa suggest that the evolution of male killing and the subsequent spread of male-killing symbionts are primarily determined by host life history characteristics. We studied the 10-spot ladybird beetle, Adalia decempunctata L. (Coleoptera: Coccinellidae), in which male killing has not been recorded previously, to test this hypothesis, and we also assessed the evolution of the male killer identified by DNA sequence analysis. Our results show that A. decempunctata harbors male-killing Rickettsia (α-proteobacteria). Male-killing bacteria belonging to the genus Rickettsia have previously been reported only for the congeneric two-spot ladybird beetle, Adalia bipunctata L. Phylogenetic analysis of Rickettsia DNA sequences isolated from different populations of the two host species revealed a single origin of male killing in the genus Rickettsia. The data also indicated possible horizontal transfer of symbionts between host species. In addition, A. bipunctata is known to bear at least four different male-killing symbionts in its geographic range two of which coexist in the two locations from which A. decempunctata specimens were obtained for the present study. Since only a single male-killing taxon was found in A. decempunctata, we assume that the two closely related ladybird beetle species must differ in the number and/or geographic distribution of male killers. We discuss the importance of these findings to our understanding of the evolution and dynamics of symbiotic associations between male-killing bacteria and their insect hosts.     

Parasite diversity and the evolution of diploidy, multicellularity and anisogamy

It may be reasonably assumed that a diversity of parasite genotypes in any one cell or organism is more harmful than a population of uniform genotypes. If this is accepted the following consequences follow: (i) Parasite mixing, due to cytoplasm mixing, at the time of zygote formation is a new and additional cost of sex. The rapid divisions typical of zygotic cleavage may be viewed as an adaptation to minimize the degree of mixing of parasites in each daughter cell. The faster the divisions the less chance parasite populations have to grow and mix. Mitosis is the fastest form of cell division. Prolongation of the diploid phase follows as a consequence of mitosis in a diploid zygote. This view is unusual in that it demands no advantage per se to the possession of two chromosome sets. (ii) The cells of the blastula formed from rapid zygotic divisions are different as regards their symbiotic inclusions. If the right to gametogenesis is restricted, then every replicator symbiont and nuclear genome alike and hence every cell of the developing embryo, will have an incentive to compete. Selection between the clonal blastula cells would result in the cells of low parasite diversity forming the gametes. Thus, germ line restriction is in the interests of the nuclear genome. Controlling the right to gametogenesis is only possible if the blastula remains intact. Hence, multicellularity might have evolved so as to enable the limitation of the right to gametogenesis and hence reduce the parasite diversity of gametes. Inter-cell competition during embryogenesis is central to Buss's seminal notion of the evolution of developmental complexity within the metazoa. The above theory provides the missing motive force behind such competition. (iii) For a given zygote size, the fittest zygotes are those produced by the gametes most disparate in size because these have a lower diversity of parasites. This may be the advantage of anisogamy. The novelty of this new view of anisogamy is that it puts a premium on sperm being very small, in order to exclude parasites from sperm cytoplasm. The hypothesis is briefly tested by examining if there are alternative means of parasite limitation in organisms with large gametes.