Sexual Plant Reproduction 14(2): 77-83 (Sept 2001)
The performance of microgametophytes is affected by inbreeding depression and hybrid vigor in the sporophytic generation
A. G. Stephenson, C. N. Hayes, M. H. Jóhannsson, J. A. Winsor

Abstract: Inbreeding reduces the level of heterozygosity, thereby exposing deleterious recessives to selection and simultaneously reducing the number of loci expressing heterosis (overdominance). In contrast, hybridization increases the level of heterozygosity, thereby masking deleterious recessives and simultaneously increasing the number of loci expressing heterosis. Most studies of inbreeding depression/hybrid vigor have focused on sporophytic performance such as survivorship, vegetative growth rates, fruit and seed production and (rarely) pollen production. Because the genetic mechanisms that underlie inbreeding depression/hybrid vigor are relevant only to the diploid stage of the life cycle, most studies have tacitly assumed that they have no effects on pollen performance (pollen germination, pollen tube growth rate, ability to achieve fertilization under conditions of pollen competition). However, we reasoned that because pollen is dependent upon the sporophyte for the resources necessary to develop, germinate and initiate tube growth, the level of heterozygosity (vigor) in the pollen-producing parent can affect pollen performance by affecting the ability of the sporophyte to provision its pollen. In a series of studies conducted under field conditions over 7 years, we experimentally varied the level of heterozygosity in wild gourd (Cucurbita pepo) plants (four levels of inbreeding, f = 0.75, 0.50, 0.25, 0 and a zucchini x wild gourd F1). We found that sporophytic vigor (e.g., flower and fruit production) increased with the level of heterozygosity and that the level of heterozygosity of the sporophyte affects the in vitro and in vivo performance of the microgametophytes it produces. These findings are analogous to the "maternal environmental effects" frequently observed in seeds.

An analysis of variance (proc GLM, SAS 1990) revealed that the F1 and two parental types of plants differed significantly in average pollen tube growth (see Jˇhannsson and Stephenson 1998b). On average, the in vitro pollen tube lengths of the F1 hybrids were 15% greater than those of zucchini and 10% greater than those from the wild gourd.

Jóhannsson MH, Stephenson AG (1998b) Variation in sporophytic and gametophytic vigor in wild and cultivated varieties of Cucurbita pepo and their F1 and F2 generations. Sex Plant Reprod 11:265–271