CYTOPLASMIC FUSION AND THE NATURE OF SEXES

Binary mating types are proposed to arise in a three-stage process thr ough selection of nuclear genes to minimize cytoplasmic gene conflict at the time of gamete fusion. In support of this view we argue that: ( i) in systems with fusion of gametes, the mating type genes are typica lly binary and regulate cytoplasmic inheritance; (ii) binary sexes hav e evolved several times independently associated with fusion, although at least twice binary types have been lost, associated with a loss of fusion; further, in accordance with the theory are findings for isoga mous species that (iii) close inbreeding may correlate with less than two sexes and biparental inheritance of cytoplasmic genes; and (iv) sp ecies with more than two sexes may have uniparental inheritance of cyt oplasmic genes, be rare and be afflicted by deleterious cytoplasmic ge nes which attempt to pervert normal cytoplasmic genetics. Such facts a nd their rationale support a new and unified definition of sexes based on the control of the inheritance of cytoplasmic genes. For the commo n cases, the male sex is that which resigns attempts to contribute cyt oplasmic genes to the next generation. We differentiate between sexes and the incompatibility types of ciliates, basidiomycetes, some angios perms and a few other organisms which are independent of organelle con tribution.