Apomeiosis circumvents sterility in inter-subspecific hybrids of rice

F₁ hybrids are known for their superior yield and greater yield stability which are due to the phenomenon of hybrid vigor, also called heterosis. In Asian rice, hybrids between compatible lines of the two subspecies indica and japonica benefit from a higher yield heterosis than intra-subspecific hybrids. However, the vast majority of indica/japonica hybrids generally exhibit partial fertility, which currently prevents their large-scale exploitation as F₁ cultivars. Recently, synthetic apomixis has emerged as an alternative to male sterility systems to produce F₁ hybrids that can be clonally propagated by seeds over generations. The most efficient synthetic apomixis system to date relies on the conversion of meiosis into mitosis (apomeiosis) by the induction of the triple Mitosis instead of Meiosis (MiMe) mutations, followed by the triggering of parthenogenesis by egg cell-specific expression of the BABY BOOM 1 (BBM1) transcription factor. We show here that apomeiosis restores full fertility in primary transformants of two indica/japonica F₁ hybrids, normally exhibiting 5.5 and 55% grain filling. We further demonstrate that the restored fertility can be transmitted to the next generation in diploid clonal progeny plants through synthetic apomixis. Our results also indicate that a better tuning of parthenogenesis is likely needed to reach both full fertility and full penetrance of apomixis in hybrid rice. Altogether, these results open new avenues for reproducing distant hybrids with high level of heterosis by clonal seeds in rice and in other crops.