The twofold cost of sex reconsidered: Meiotic mechanisms protect anisogamous populations from invasion by thelytoky

Most multicellular animals practice anisogamy (fertilization between eggs and sperm). When mothers produce sons and daughters at a 1:1 ratio, the "twofold cost of males" arises because males do not directly contribute to population growth. If thelytokous parthenogens producing only daughters invade a population, they should spread rapidly. Although thelytoky has repeatedly evolved across invertebrate and vertebrate taxa, it remains a minority. Why? The evolutionary transition from anisogamy to thelytoky requires eggs to initiate embryonic development without fertilization. However, in metazoan animals, meiotic metaphase (MM) arrest halts oogenesis midway and normally resumes only after stimulation by sperm penetration. Empirical and experimental evidence indicates that release of MM arrest without fertilization is extremely difficult, providing a strong mechanistic barrier against parthenogenesis. Even if MM arrest were released, oogenesis would proceed to produce either a haploid embryo or a diploid embryo through refusion with the second polar body (terminal fusion automixis). Outbred species typically accumulate more than one lethal equivalent of recessive deleterious alleles per genome as heterozygotes. Upon transition to haploid or automictic development, these recessive lethals normally masked in outbred diploids would be exposed simultaneously, causing embryonic death and creating the next barrier. Thus, thelytoky cannot be achieved simply by modification of the existing meiotic system; instead, other mechanisms, such as apomixis, that bypass meiosis are required. Mathematical models and simulations support this "meiotic constraint" hypothesis. Combined with recently proposed immediate benefits of anisogamy and traditional genetic benefits (e.g., Red Queen), it may largely explain the maintenance of costly anisogamy.