High hybrid fitness at seed and seedling life history stages in Louisiana irises

1 Relative fitness of hybrid genotypes will determine the role that hybrids can play in the evolution of a plant species complex. To realistically evaluate hybrid fitness and how environmental variation affects fitness in a long-lived species, all life stages must be considered. 2 We evaluated germination, seedling survival and growth of two Louisiana iris species and their early generation hybrids in several experimental environments created by manipulating light and water levels. 3 Species and hybrids all required similar moist conditions for germination. The proportion of germinated seeds was highest in shade and seedling survival highest in sun. 4 Iris brevicaulis exhibited the lowest germination and seedling survival overall, yet those individuals that survived grew vigorously Iris fulva had high levels of germination and seedling survival, but yielded the smallest plants at the end of one season of growth. 5 Germination rates, seedling survival and seedling growth of hybrids equalled or exceeded one or both parent species, indicating that hybrids in this system have high relative fitness at seed and seedling stages in several environmental conditions. 6 We conclude that Iris fulva and I. brevicaulis share a common regeneration niche and similar early stage relative fitness with their hybrids. Thus, environment-dependent fitness in this system is due primarily to selection at adult stages.