Divergence in flowering time is a major component contributing to reproductive isolation between two wild rice species (Oryza rufipogon and O. nivara)

Abstract

It is of critical importance for our understanding of speciation process to determine the forms of reproductive isolation and their relative importance in species divergence. Oryza nivara and O. rufipogon are direct ancestors of Asian cultivated rice and a progenitor-daughter species pair. Investigating the reproductive isolation between them provides insights into plant speciation and helps understanding of the rice domestication. Here, we quantitatively measured the major components of reproductive isolation between the two species based on common garden and crossing experiments for three pairs of sympatric populations in Nepal, Cambodia and Laos.We revealed significant differences in the flowering times between species pairs, with O. nivara flowering much earlier than O. rufipogon. A very weak reduction in seed set but no reduction in F1 viability and fertility were detected for the crosses between species relative to those within species. Moreover, we detected asymmetrical compatibility between species and found that emasculation significantly decreased pollination success in O. nivara but not in O. rufipogon. Our study demonstrates that the divergence between O. nivara and O. rufipogon is maintained almost entirely by the difference in flowering times and suggests that differential flowering times contribute to both habitat preferences and reproductive isolation between species.

Crossability analysis of cultivated rice (Oryza sativa) with O. rufipogon and O. longistaminata and F1-identification

“…longistaminata were further observed and could be differentiated from parents in terms of several qualitative and quantitative traits. It has also been suggested that this diversity in the reproductive isolation is mainly due to diversity in flowering time (Xu et al 2020). Hybrids were found intermediate between the two parents for many distinguishing traits.…”

Rice functional genomics: decades’ efforts and roads ahead

“…Suitable heading date of rice is positively correlated with grain yield: the longer the heading date is, the more biomass and the higher productivity are obtained (Zhang et al, 2019a). The genetic divergence affecting flowering time is also a major factor of reproductive isolation between Oryza species (Xu et al, 2020). Heading date is controlled not only by certain genes but also by some external factors such as photoperiod and temperature.…”

Experimental sympatry suggests geographic isolation as an essential reproductive barrier between two sister species of Pedicularis

“…RI can be classified as pre-or post-zygotic, according to the timing in relation to stages of reproduction (Coyne & Orr, 2004). Pre-zygotic isolation may involve spatial (geographical isolation) and temporal separation (phenological isolation) (Lowry et al, 2008a(Lowry et al, , 2008bSobel & Streisfeld, 2015;Xu et al, 2020), use of different pollinators or mechanical and ethological isolation mediated by a common pollinator (floral isolation, Grant, 1994b), or pollen-pistil interactions in flowering plants (Fishman et al, 2008;Liang et al, 2018). As for post-zygotic barriers, low viability or sterility of interspecific hybrids has usually been observed (Yang et al, 2007;Martin et al, 2017;Christie & Strauss, 2019).…”