Academic journal article Genetics

A Genetic Model for the Female Sterility Barrier between Asian and African Cultivated Rice Species

Academic journal article Genetics

A Genetic Model for the Female Sterility Barrier between Asian and African Cultivated Rice Species

Article excerpt

ABSTRACT

S^sub 1^ is the most important locus acting as a reproductive barrier between Oryza sativa and O. glaberrima. It is a complex locus, with factors that may affect male and female fertility separately. Recently, the component causing the allelic elimination of pollen was fine mapped. However, the position and nature of the component causing female sterility remains unknown. To fine map the factor of the S^sub 1^ locus affecting female fertility, we developed a mapping approach based on the evaluation of the degree of female transmission ratio distortion (fTRD) of markers. Through implementing this methodology in four O. sativa × O. glaberrima crosses, the female component of the S^sub 1^ locus was mapped into a 27.8-kb (O. sativa) and 50.3-kb (O. glaberrima) region included within the interval bearing the male component of the locus. Moreover, evidence of additional factors interacting with S^sub 1^ was also found. In light of the available data, a model where incompatibilities in epistatic interactions between S^sub 1^ and the additional factors are the cause of the female sterility barrier between O. sativa and O. glaberrima was developed to explain the female sterility and the TRD mediated by S^sub 1^. According to our model, the recombination ratio and allelic combinations between these factors would determine the final allelic frequencies observed for a given cross.

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INTRINSIC postzygotic reproductive barriers are a very common and important phenomenon, driving the establishment and conservation of species (Widmer et al. 2009). They are observed as a reduction in the hybrids' fitness, evidenced by a reduced viability or fertility. So far, a limited number of genes acting as postzygotic barriers have been identified in Drosophila (Ting et al. 1998; Presgraves et al. 2003; Brideau et al. 2006; Masly et al. 2006; Phadnis and Orr 2009; Tang and Presgraves 2009), and in other animal systems (Wittbrodt et al. 1989; Lee et al. 2008; Mihola et al. 2009), but also in Arabidopsis (Josefsson et al. 2006; Bomblies et al. 2007; Bikard et al. 2009) and rice (Chen et al. 2008; Long et al. 2008).

In rice improvement, hybrid inviability and sterility are major obstacles for the common utilization of closely related species in breeding programs, impairing the exploitation of the rich genetic diversity found within the Oryza sativa complex (known as genome group AA), and the beneficial effect of the high level of heterosis observed in the F1 plants. One of the most relevant examples of this strong limitation comes from the African cultivated rice species O. glaberrima Steud. This species represents an interesting source of drought tolerance (Sarla and Swamy 2005), weed competitiveness (Dingkuhn et al. 1998), and nematode and virus resistances (Ndjiondjop et al. 1999; Soriano et al. 1999). Several of these traits have been already mapped (Lorieux et al. 2003; Ndjiondjop et al. 2003); however, their introduction to O. sativa has been hampered by the strong sterility barrier between the two species (Sano et al. 1979). This barrier is the result of several loci that might interact or act separately to render the F1 hybrids pollen sterile and partially female fertile. Among these, the S^sub 1^ locus has the strongest effect over the fertility of the hybrids (Sano 1990).

Despite the strong hybrid sterility, obtaining fertile plants derived from O. sativa 3 O. glaberrima crosses has been possible by performing successive backcrosses followed by selfing (Ghesquière et al. 1997). Nevertheless, this fertility recovery is associated with the presence of the homozygote O. glaberrima S^sub 1^ allele (S^sub 1^ g), and is lost again when recrossing with O. sativa (Heuer and Miezan 2003). Furthermore, a strong transmission ratio distortion (TRD) of markers linked with S^sub 1^, in favor of the O. glaberrima alleles, results as a consequence of the systematic elimination of the O. …

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