(1) The analysis described here predicts a large, rapid reduction in fitness associated with isogamous sexual reproduction, due to the spread of deleterious cytoplasmic agents with fitness only subsequently rising to a maximum twofold cost as uniparental inheritance of cytoplasm and anisogamy evolve.
(2) Alternatively, anisogamy can be explained as a side-effect of selection for a greater efficiency in finding a mating partner by using sexual pheromones.
(3) The second large category of costs is derived from anisogamy.
(4) It is suggested that these were the conditions under which anisogamy evolved.
(5) As examples, the origin of anisogamy and cyclic chases for predator-prey coevolution are studied.
(6) It is very likely that sexual differentiation into two morphologically indistinguishable mating types has preceded the evolution of anisogamy.
(7) Standard theory suggests there is no immediate cost of sex, a twofold cost being incurred later as anisogamy evolves.
(8) Nearly all theories proposed for the evolution of anisogamy assume two conflicting selection forces to be relevant: selection for greater gamete productivity, and selection for greater zygote size.
(9) 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.
(10) The first category consists of costs that are not derived from anisogamy, and includes costs associated with recombination, with delay of synthesis at the cellular level (cellular-mechanical cost), and with fertilization.
(11) Therefore, the study of the evolution of mating types in an isogamous population is more informative for understanding the forces responsible for the evolution of different sexes than the study of the evolution of anisogamy; the latter represents the secondary problem of how, after the establishment of two sexes, an increasing degree of gamete dimorphism may evolve.
(12) In both problem areas--mating type evolution and anisogamy evolution--experimental work is badly needed.
(13) This in turn would reinforce the evolution of anisogamy.
Heterogamy
Definition:
(n.) The process of fertilization in plants by an indirect or circuitous method; -- opposed to orthogamy.
(n.) That form of alternate generation in which two kinds of sexual generation, or a sexual and a parthenogenetic generation, alternate; -- in distinction from metagenesis, where sexual and asexual generations alternate.