EVOLUTION OF SEX-RATIOS IN ISOPOD, VENEZILLO-EVERGLADENSIS

Monogenic females producing families significantly departing from 1:1 sex ratios exist among terrestrial isopods, in frequencies as high as 13.8% in V. evergladensis. An infective, spirochete-like, cytoplasmic particle has been identified as responsible for the unequal ratios in sibships of individual females and is probably the general explanation for monogeny among isopods. The particle''s mode of action, while unknown, does not reduce fecundity by early mortality of specific sexes. Monogeny was earlier suggested as advantageous due to its potential for reducing inbreeding effects arising from full and half-sib matings. A breeding study of V. evergladensis for a color polymorphism contributed data for brood size and interval between broods for 4 levels of inbreeding. First generation full-sib matings yield significantly lower fecundities than for nonrelated parents as measured by brood size. In addition, intervals between the smaller broods of the full-sibs do not compensate for their lower reproductive output per brood. Half-sib matings are not significantly lower in fecundity than for matings between nonrelated parents though the data suggest such a trend. The practice of multiple matings by females reduces the number of full-sibs per family, and the dispersal of young during maturation further reduces the likelihood of any sib mating. Sex ratio estimates are available for separate sibships where mortality following birth is judged insignificant. Individual monogenic sibships differ largely from 1:1 sex ratios but occur in numbers giving a 1:1 pooled sex ratio for the totatl zygote population. Isopods have probably been unable to escape totally the spirochete particle invasion and have responded by evolution of a genetically controlled switch. The switch could control the direction taken by the sex ratio effect. Frequency-dependent selection predictably operates on females producing disproportionately larger numbers of male or female offspring where the 2 types stabilize at equal frequencies.