Interspecific seed discounting and the fertility cost of hybridization in an endangered species.
Hybrid fertilizations can have negative demographic effects on taxa by usurping ovules that would otherwise give rise to nonhybrid offspring. The consequent reduction in conspecific matings may be exaggerated in rare taxa and constitutes a fertility cost that has rarely been quantified. Here, the effect of interspecific mating was estimated on the fecundity of locally rare red mulberry (Morus rubra), which hybridizes with introduced white mulberry (Morus alba) and red yen white hybrids. First, the asymmetry in pollen production among red, white and hybrid mulberry in two sympatric populations was quantified. The fertility cost of hybridization was then assessed experimentally by estimating seed production and rates of interspecific mating in red mulberry trees from plots where white and hybrid mulberry trees were selectively removed. On average, the percentage of mulberry pollen per plot produced by red mulberry (8%) was significantly lower than the mean for white and hybrid mulberry combined (92%). Experimentally removing white and hybrid mulberry increased the siring fertility of red mulberry by 14% but produced no change in seed set. Results indicate that seeds of red mulberry, ordinarily sired by conspecific pollen, are being discounted through fertilization of ovules by heterospecific pollen, which may contribute to local decline of red mulberry.
Burgess
Kevin S
KS
Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1, Canada.|Present address: Department of Ecology & Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.
Morgan
Martin
M
School of Biological Sciences, Washington State University, PO Box 644236, Pullman, WA 99164-4236, USA.
Husband
Brian C
BC
Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
D003247:Conservation of Natural Resources; D006824:Hybridization, Genetic; D031625:Morus / Q000235:genetics / Q000502:physiology*; D011058:Pollen; D012098:Reproduction / Q000235:genetics / Q000502:physiology; D012639:Seeds