Sex ratio distorters and their detection

Sex ratio distorters (SRDs) are heritable elements that modify the sex ratio of their host to promote their own transmission. In this chapter we examine various theories relating to the evolutionary importance of SRDs, give an overview of the various classes of SRDs and methods of how to discover them in the field, and outline areas of current contention, and thus future work in relation to their incidence and importance. Sex ratio distorters include organelles, heritable bacteria and eukaryotes, B chromosomes and meiotic-drive sex chromosomes. A high proportion of arthropod species that have been studied in detail harbour SRDs. They are important in host evolution because they influence fundamental population dynamic processes, manipulate sex-determining mechanisms of their host and may contribute to genetic isolation between host populations. If SRDs are parasitic, selection may promote the spread of host genes to prevent SRD action and transmission. Unusual sex ratio phenotypes in the field may indicate SRDs. This should be followed by genetic analysis of sex ratio phenotypes of isofemale lines. If micro-organisms are suspected, they can be identified molecularly with specific polymerase chain reaction (PCR) primer pairs and sequence analysis. Introduction What are sex ratio distorters (SRDs)? SRDs are broadly defined as those heritable elements that modify the sex ratio of their host to promote their own transmission, often at a cost to the inclusive fitness of the individual bearing them.