Prezygotic Reproductive Isolation and Fertility in Crosses between <italic>Indica</italic> and <italic>Japonica</italic> Subspecies

Abstract

Reproductive isolation is essential to the formation and maintenance of new species. Understanding the mechanism of reproductive isolation is fundamental in biological science. Reproductive isolation occurred between the indica and japonica subspecies in rice. However, mechanisms of prezygotic reproductive isolation in rice are poorly understood, which require further investigation. In this study, we have investigated the adherence and germination of the pollens using 17 crosses from a total of 12 varieties representative of indica , japonica , and wide-compatibility varieties. Prezygotic reproductive isolation was observed between the indica and japonica subspecies, which showed that the average pollen adhered was less than 5 per stigma in 2/3 inter-subspecific crosses. However, the average number of adherent pollen was more than 5 per stigma in 3/4 intra-subspecific crosses and all the crosses with the wide-compatibility varieties. Various abnormalities of pollen tube growth were observed in inter-subspecific crosses, but not in intra-subspecific crosses and crosses with the wide-compatibility varieties. The amount of the pollen adhered was irrelevant to the fertility in F1 hybrids, indicating that prezygotic and postzygotic reproductive isolation occurred independently. The reduced affinity between the pollens and stigma in indica and japonica subspecies prevents the formation of hybrid zygotes, which contributes to prezygotic reproductive isolation in rice. Our findings provide valuable insight in understanding the mechanism of reproductive isolation and have implications for utilization of heterosis.