Evol Ecol Res 2: 745-759 (2000)     Full PDF if your library subscribes.

Sex ratio and brood size in a monophagous outcrossing gall aphid, Tamalia coweni (Homoptera: Aphididae)

Donald G. Miller III1,2,3 and Leticia Avilés2

1Department of Biology, Trinity University, 715 Stadium Drive, San Antonio, TX 78212, 2Department of Ecology and Evolutionary Biology and 3Center for Insect Science, University of Arizona, Tucson, AZ 85721, USA

Address all correspondence to Donald G. Miller III, Department of Biology, Trinity University, 715 Stadium Drive, San Antonio, TX 78212, USA.
e-mail: dmiller@trinity.edu


Sex allocation theory has been applied successfully in the case of spatially structured aphid populations, in which local mate competition can account for biased sex ratios. Likewise, a demographic effects model can explain sex ratio bias when maternal investment in sons and daughters is asynchronous, owing to developmental constraints. In one of the first studies to examine patterns of sex allocation in a phytophagous insect in which outbreeding is likely and both sexes are produced concurrently, we measure sex ratio and sex allocation for the gall-forming aphid Tamalia coweni (Cockerell). While the sex ratio at a low elevation (800 m) site did not differ significantly from 1 : 1, the sex ratio was slightly but significantly female-biased at a higher elevation (1350 m). At both sites, the variance in sex ratio among broods was significantly greater than binomial, suggesting active manipulation of the sex ratio by aphid foundresses. Ten percent of the broods with more than four individuals contained exclusively males or exclusively females, a percentage that could not have resulted from random variation around an average sex ratio. Among non-unisex broods, the sex ratio became increasingly more female-biased with increasing brood size. Local mate competition and non-adaptive demographic effects on the sex ratio could not account for the overall bias towards females. Apart from the possibility of cytoplasmic or other genetic sex ratio distortion elements, the Trivers-Willard hypothesis of condition-dependent sex allocation may best explain the observed sex ratio patterns in T. coweni.

Keywords: brood size, gall aphid, sex allocation, sex ratio, Tamalia coweni.

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