Title

Sex-Ratio In The Sailfin Molly, Poecilia-Latipinna

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Abbreviated Journal Title

Evolution

Keywords

Ecology; Evolutionary Biology; Genetics & Heredity

Abstract

R. A. Fisher's theory predicts that the sex ratio at conception and birth should be 1.1 in most diploid, bisexual organisms. This ratio should not be directly influenced by differential mortality or other factors acting to bias sex ratios away from 1.1 among adults. Most live-bearing fishes of the family Poeciliidae exhibit female-dominated adult sex ratios. The sex ratios among young and juveniles are well-known for only one species, Gambusia affinis. For this species, the secondary sex ratio is 1.1, but females predominate among adults. We examined sex ratios in the sailfin molly, Poecilia latipinna, on the east-central coast of Florida. The sex ratio at birth was examined in lab-reared broods. Sex ratios among juveniles and adults were recorded from two populations sampled monthly for two years. The sex ratio was not significantly different from 1.1 in 22 of the 26 broods reared. In four broods, females predominated significantly. Overall, 291 of 568 neonates (51.2%) were females. Apparently, the sex ratio in a small number of broods may depart significantly from unity; but among all neonates in a cohort, the sex ratio is very near 1.1. In field populations, the sex ratio among small (juvenile) mollies was not significantly different from 1:1 in 32 of 47 monthly samples. In samples where departure was significant, there was no clear temporal or spatial pattern and no clear explanation for the direction or degree of skewness. Overall, females constituted 47-54% of about 10,000 small fish examined. Except for January, 1974, at RRCD, the sex ratio among adults from field samples was highly skewed in favor of females at both study sites. The departure from 1:1 was statistically significant in 44 of 46 cases. Overall, 72% of over 17,000 adult fish were females. In some months females constituted 80-90% of the adult fish captured. Our results support Fisher's theory. Despite highly skewed sex ratios in adults, males and females occur in approximately equal numbers among young and juveniles. Differential mortality of males appears to coincide with the initiation of sexual maturation. Several proximate causes of differential mortality have been suggested. Ultimately, differential male mortality must be adaptive since it seems to be a general phenomenon in the family Poeciliidae. We discuss the significance of unbalanced sex ratios in poeciliid reproductive systems and suggest two alternative hypotheses for how skewed ratios may have evolved.

Journal Title

Evolution

Volume

34

Issue/Number

2

Publication Date

1-1-1980

Document Type

Article

Language

English

First Page

308

Last Page

319

WOS Identifier

WOS:A1980JQ65100011

ISSN

0014-3820

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