SEX ALLOCATION AND OUTCROSSING RATE: A TEST OF THEORETICAL PREDICTIONS USING BROMEGRASSES (BROMUS).

Predictions of sex-allocation theory were tested by comparisons among hermaphroditic bromegrass (Bromus) species that differed in outcrossing rate. Relative maternal and paternal investment were calculated using both the ratio of pollen to seed production, and absolute allocations in units of energy, nitrogen, phosphorus, potassium, magnesium, and calcium. Outcrossing rate had a large effect on sex allocation; species having greater outcrossing rates had relatively more paternal reproductive effort. Bromus inermis was obligately outcrossing, and nearly half of its reproductive effort was devoted to pollen production. Three partially outcrossed species, B. kalmii, B. ciliatus, and B. latiglumis, invested between 5% and 11% of reproductive effort in pollen production. Paternal investment was less than 2% in the selling species B. tectorum. Estimates of sex allocation were relatively unaffected by the resource currency used in calculation. The differences among species in sex allocation were mostly due to differences in anther size and seed set.>. © 1987 The Society for the Study of Evolution.