Data from: Lineages of Silene nutans developed rapid, strong, asymmetric postzygotic reproductive isolation in allopatry

Reproductive isolation can rise either as a consequence of genomic divergence in allopatry or as a byproduct of divergent selection in parapatry. To determine whether reproductive isolation in gynodioecious Silene nutans results from allopatric divergence or from ecological adaptation following secondary contact, we investigated the pattern of postzygotic reproductive isolation and hybridization in natural populations using two phylogeographic lineages, western (W1) and eastern (E1). Experimental crosses between the lineages identified strong, asymmetric postzygotic isolation between the W1 and the E1 lineages, independent of geographic overlap. The proportion of ovules fertilized, seeds aborted, and seeds germinated revealed relatively little effect on the fitness of hybrids. In contrast, hybrid mortality was high and asymmetric: while half of the hybrid seedlings with western lineage mothers died, nearly all hybrid seedlings with E1 mothers died. This asymmetric mortality mirrored the proportion of chlorotic seedlings, and is congruent with cytonuclear incompatibility. We found no evidence of hybridization between the lineages in regions of co-occurrence using nuclear and plastid markers. Together, our results are consistent with the hypothesis that strong postzygotic reproductive isolation involving cytonuclear incompatibilities arose in allopatry. We argue that the dynamics of cytonuclear gynodioecy could facilitate the evolution of reproductive isolation.

生殖隔离 (reproductive isolation) 既可以作为异域分布下基因组分化的结果而产生，也可作为邻域分布下分化选择的副产物形成。为明确雌全异株 (gynodioecious) 垂序麦瓶草 (Silene nutans) 的生殖隔离源于异域分化，还是次级接触后的生态适应，我们以两个系统地理支系 (phylogeographic lineages)——西部支系 (W1) 与东部支系 (E1)——为研究对象，探究了自然种群中的合子后生殖隔离 (postzygotic reproductive isolation) 模式与杂交情况。跨支系的杂交实验 (experimental crosses) 显示，W1与E1支系间存在强烈的不对称合子后生殖隔离，且与地理重叠无关。对受精胚珠 (ovules fertilized) 比例、败育种子 (seeds aborted) 比例及萌发种子 (seeds germinated) 比例的分析显示，上述因素对杂种适合度的影响相对微弱。与之形成鲜明对比的是，杂种死亡率 (hybrid mortality) 颇高且呈现不对称特征：以西部支系为母本的杂种幼苗约有半数死亡，而以东部支系为母本的杂种幼苗则几乎全部死亡。这种不对称的死亡率与失绿幼苗 (chlorotic seedlings) 的占比高度一致，且符合核质互作不亲和 (cytonuclear incompatibility) 的理论特征。我们借助核基因标记与质体标记 (nuclear and plastid markers)，未在两支系同域分布 (co-occurrence) 的区域发现杂交证据。综上，本研究结果支持如下假说：涉及核质互作不亲和的强合子后生殖隔离，是在异域环境中演化形成的。我们认为，核质互作型雌全异株的动态调控机制，可推动生殖隔离的演化。