BARRIERS TO HYBRIDIZATION OF TRIFOLIUM REPENS WITH RELATED SPECIES

Abstract

Pre-fertilization barriers to hybridization of Trifolium repens L. with its related species, T. nigrescens, T. occidentale, T. hybridum, T. ambiguum, and T. uniflorum, were studied by comparing pollen germination, pollen-tube growth, and fertilization following intra- and interspecific pollinations on T. repens. Pollen of T. repens began to germinate almost immediately after being placed on the stigmas of its own species; pollen germination approached 100% in 30 min. Pollen tubes grew rapidly. Eight hours after pollination the longer tubes had entered the micropyle of the most distant ovules. Fertilization was first observed at 8 hr. All ovules were fertilized within 24 hr. In interspecific pollinations using T. repens as the pistillate parent, the time required for pollen germination was longer and the frequency of germination was lower. Pollen tubes frequently swelled, coiled, or even burst in the styles. Pollen tubes of the species more closely related to T. repens appeared to grow faster and more normally. Fertilization occurred in all interspecific crosses. The highest frequency of ovules fertilized generally occurred in the species combinations in which pollen-tube growth was most similar to the control and the lowest frequency, in the combinations in which tube growth deviated the most from the control. These observations suggest that pre-fertilization barriers are not the only causes of cross-incompatibility of T. repens with related species. Post-fertilization barriers, which prevent zygotes from developing into viable seeds, also must exist.