SEX-RATIO BIAS WITH ASYMMETRIC EXCHANGE OF POLLEN BETWEEN DEMES

In a patch of hermaphroditic plants, with a low level of pollen migrat ion between patches, a prevailing wind creates a gradient, within the patch, in the strength of local competition among pollen for reproduct ive success. a This leads to a sex ratio gradient, with a male (pollen ) bias in downwind individuals, which can be quite strong even for lar ge patches. The effect can be understood as follows: downwind individu als have relatively low reproductive value and respond by putting more resources into the gamete (pollen) with the best long-range (extra-pa rch) reproductive success.