Hybrid sterility between Asian and African rice can be mitigated by tetraploidization

Related Research Project

Tropical crop genetic resources

Country

Africa

Description

Almost all rice varieties cultivated worldwide belong to the Asian rice (Oryza sativa), and modern rice varieties with high yielding capacity and good eating quality are developed through intraspecific crosses within O. sativa. In contrast, the African rice (O. glaberrima), cultivated only in limited regions of West Africa, exhibits distinct traits such as resistance to certain pests and diseases and adaptability to low-fertility soils. Hybridizing these two species may enable the development of varieties that can be cultivated in environments where O. sativa cultivation has been difficult. However, interspecific F₁ hybrids between O. sativa and O. glaberrima exhibit severe sterility of pollen grains (hybrid sterility), preventing seed production; thus, it is essential to develop fertile interspecific hybrids to overcome this sterility.

In this work, we investigated the relationship between hybrid sterility and ploidy level of the hybrid and found that tetraploid F₁ hybrids can restore pollen fertility and set seeds by self-pollination.

We examined the pollen fertility of F₁ hybrids between an O. glaberrima variety (Og) and three O. sativa varieties (Os1, Os2, and Os3 belonging to the temperate japonica, indica, and aus subspecies, respectively), and found that while diploid F₁ hybrids showed 0% pollen fertility (complete sterility), tetraploid F₁ hybrids exhibited pollen fertility ranging from 5.7% to 28.1%, enabling self-pollination (Fig. 1).

Genetic analysis of the fertile microspores in the tetraploid hybrids provided insight into the fertility restoration mechanism (Fig. 2). For example, in the S2 locus, which is one of the loci inducing pollen grain sterility in the hybrid, all microspores carrying the S2 allele from O. glaberrima would be sterile in diploid hybrids. However, they could maintain fertility in the tetraploid situation by carrying the S2 allele from O. sativa simultaneously.

These results suggest that employing tetraploid hybrids can facilitate genetic exchange between the two species, allowing for the development of diverse hybrids with valuable traits, which was difficult in the diploid situation. Furthermore, diploid hybrids between the two species could be developed successfully by rescuing the fertile microspores in the tetraploid hybrid through anther culture methods. Further investigation on pollen fertility and agronomic traits of these diploid hybrids will be required to estimate their usability as hybrids with intermediate genomic compositions between the two species.