Reproductive isolation among lineages of Silene nutans (Caryophyllaceae): A potential involvement of plastid-nuclear incompatibilities

Early stages of speciation in plants might involve genetic incompatibilities between plastid and nuclear genomes, leading to inter-lineage hybrid breakdown due to the disruption between co-adapted plastid and nuclear genes encoding subunits of the same plastid protein complexes. We tested this hypothesis in Silene nutans, a gynodioecious Caryophyllaceae, where four distinct genetic lineages exhibited strong reproductive isolation among each other, resulting in chlorotic or variegated hybrids. By sequencing the whole gene content of the four plastomes through gene capture, and a large part of the nuclear genes encoding plastid subunits from RNAseq data, we searched for non-synonymous substitutions fixed in each lineage on both genomes. Lineages of S. nutans exhibited a high level of dN/dS ratios for plastid and nuclear genes encoding most plastid complexes, with a strong pattern of coevolution for genes encoding the subunits of ribosome and cytochrome b6/f that could explain the chlorosis of hybrids. Overall, relaxation of selection due to past bottlenecks and positive selection have driven the diversity pattern observed in S. nutans plastid complexes, leading to plastid-nuclear incompatibilities. We discuss the possible role of gynodioecy in the evolutionary dynamics of the plastomes through linked selection.