PREDICTING EVOLUTIONARY CONSEQUENCES OF GREATER REPRODUCTIVE EFFORT IN TRIPSACUM-DACTYLOIDES, A PERENNIAL GRASS

To test the prediction that a mutation causing greater reproductive ef fort will result in reduced vegetative vigor, we compared the seed pro duction, growth, and carbohydrate status of normal and pistillate geno types of the grass Tripsacum dactyloides differing in seed production by up to fourfold. We evaluated the costs of reproduction by two metho ds: experimental manipulation of reproductive effort and comparison of high- and low-yielding genotypes. Despite the large difference in see d production, normal and pistillate (high-yielding) genotypes did not differ in growth rate over a 1-yr period. Contrary to predictions, car bohydrate reserves in the high-yielding genotype were significantly hi gher at the peak of the seed ripening period. Stalk defoliation and co mplete removal of seed stalks reduced plant growth rate, total abovegr ound biomass, and biomass of vegetative storage organs, especially whe n plants were also under stress from biweekly defoliation. However, st alk removal caused increased rates of growth in the year after experim ental manipulations. Reproductive tillers were costly to the plant in terms of lost meristems and therefore future plant size, but seed cost s were contained within reproductive tillers of both genotypes, explai ning the lack of a trade-off in the high-yielding, pistillate genotype . Although experimental reduction of reproductive effort revealed a co st of reproduction, this result could not be used to correctly predict the consequences of a gene for greater seed production.