The male gametes of most organisms lack cytoplasm. Consequently, most
cytoplasmic genetic elements are maternally inherited: they cannot be
transmitted patrilinnearly. The evolutionary interests of cytoplasmic
elements therefore lie in transmission through the female. These eleme
nts may thus be in evolutionary conflict with nuclear genes which are
transmitted by both sexes. This conflict is manifested in observations
of cytoplasmically induced biased sex-ratios. Some cytoplasmic genes
avoid this fate by biasing the primary sex ratio towards females, or b
y inducing parthenogenesis. Others kill male hosts, and either achieve
transmission via dispersal, or benefit their clonal relatives in the
dead male's female siblings. Still others cause the failure of zygotes
resulting from pairings between males carrying specific microbes and
females lacking them, causing an increase in the microbes through the
sterilisation of non-bearing females. Many, but not all, of these 'ult
ra-selfish' microbes are closely related. Investigations of the signif
icance of their phylogenetic affinities, or lack of them, their adapta
bility in terms of the methods by which they avoid, or ameliorate, the
adverse effects of being in male hosts, and their importance as selec
tive agents in the evolution of invertebrate sex determination systems
, provide fertile spheres for future research.