The interplay of hybridization and clonal reproduction in the evolution ofwillows - Experiments with hybrids of S-eriocephala [R] & S-exigua [X] andS-eriocephala & S-petiolaris [P].

Clonal reproduction may contribute to plant evolution either by affecting p opulation biology or by allowing partially sterile individuals (e.g., hybri ds) repeated opportunities to reproduce sexually. The interplay of hybridiz ation and clonal reproduction was first proposed by Stebbins (1950), but pr eviously has not been tested experimentally. Alternative models for the eff ects of hybridization include speciation, introgression, and swamping. Sali x spp. were chosen to test comparatively these hypotheses because they are easily hybridized and cloned. Over 500 separate crosses and backcrosses wer e made and over 6000 separate plants were measured in field experiments and statistically compared for significance to both evolutionary theory and pl ant breeding for biomass production. The F-1 hybrids in this study always e qualled, and in the case of the hybrid PR, outperformed their parents in ve getative parameters. It seems likely that even without reproducing sexually , these F-1 hybrids could exist as successful individuals (sensu Stebbins 1 950). However, it also seems likely that they would sexually reproduce: thr ee of four F-1 hybrids studied (RX, RP, and PR) equalled or surpassed their parents in sexual parameters when crossing with at least one other accessi on. Of the alternative models, experimental data suggest that introgression would be the most likely outcome of a hybridization event. The hybrid XR, however, was partially sterile and performed poorly when crossing with all other accessions in its group except S. exigua (pistillate parent). Thus, t his hybrid may fit Stebbins' model of a partially sterile yet vegetatively vigorous plant that can exist as a successful individual and make some cont ribution to interspecific gene flow over time. This is the first experiment al study to confirm the evolutionary importance of clonal reproduction coup led with hybridization. However, distinguishing any of these evolutionary p athways would be difficult in nature using morphological techniques, as int erspecific hybrids tend to resemble their pistillate parents in terms of le af shape.