Seed weight and seed number affect subsequent fitness in outcrossing and selfing Primula species

Using the outcrossing Primula farinosa and its autogamous selfing relatives P. scotica, P. scandinavica and P. stricta, we compared the fitness of lig ht and heavy seeds. Heavy seeds germinated in greater numbers and more quic kly. In competition with seedlings grown from lighter seeds, heavy seeds pr oduced larger rosettes. In P. farinosa such seedlings went on to produce mo re seeds, and in two populations heavier seeds, than plants from lighter se eds. After transplantation to natural populations, seedlings of P. farinosa derived from heavy seeds produced larger rosettes, more flowers and seeds than those from lighter seeds in certain populations so that seedlings born of heavy seeds were much fitter than seedlings from lighter seeds. Average seed weight varied in inverse proportion to seed number per capsule. The a utogamous species produced on average about twice as many seeds per capsule as P. farinosa. In P. scotica and P. stricta this difference appears to be due in part to assured fertilization, but this high fecundity did not caus e disadvantageously light seeds. As these species produced fewer capsules p er scape, their overall seed production was on average no greater than for P. farinosa. P. farinosa traded-off fitness between capsules with large see d numbers, which donated more offspring to the next generation, and those w ith small seed numbers, whose heavy seeds would be more likely to reproduce themselves in the next generation. We conclude that low fecundity in outcr ossing species might at times be advantageous.