Data from: Incomplete loss of a conserved trait: function, latitudinal cline, and genetic constraints

Retention of nonfunctional traits over evolutionary time is puzzling, because the cost of trait production should drive loss. Indeed, several studies have found nonfunctional traits are rapidly eliminated by selection. However, theory suggests that complex genetic interactions and a lack of genetic variance can constrain evolution, including trait loss. In the mustard family Brassicaceae the conserved floral condition includes four long and two short stamens, but we show that short stamens in the highly self-pollinating mustard Arabidopsis thaliana do not significantly increase selfed seed set, suggesting that the trait has lost most or all of its function after the transition to selfing. We find that short stamen loss is common in native populations. Loss is incomplete and decreases with increasing latitude, a cline unexplained by correlations with flowering time or ovule count (which also vary with latitude). Using recombinant inbred lines derived from a cross between plants at the latitudinal extremes of the native range, we found three QTLs affecting short stamen number, with epistasis among them constraining stamen loss. Constraints on stamen loss from both epistasis and low genetic variance may be augmented by high selfing rates, suggesting that these kinds of constraints may be common in inbred species.演化时间尺度上非功能性性状的保留始终令人困惑，因为性状产生的成本理应推动其丢失。此前多项研究已证实，非功能性性状会被自然选择快速清除。不过理论研究表明，复杂的遗传互作与遗传变异的匮乏会限制演化进程，其中也包括性状丢失过程。在十字花科（Brassicaceae）植物中，保守的花部构型包含4枚长雄蕊与2枚短雄蕊。本研究发现，在高度自花授粉的拟南芥（Arabidopsis thaliana）中，短雄蕊并未显著提升自交种子的结实率，表明该性状在演化转向自交繁育系统后，已丧失了全部或绝大多数功能。我们在自然种群中发现，短雄蕊丢失的现象十分普遍，但该丢失过程并不彻底，且随纬度升高而逐渐减弱；这种纬度渐变群现象无法通过与开花时间或胚珠数（二者同样随纬度发生变化）的相关性得到解释。我们利用源自原生分布区纬度两端植株杂交得到的重组自交系群体开展分析，鉴定出3个影响短雄蕊数量的数量性状位点（Quantitative Trait Locus，QTL）；位点间的上位性互作会限制短雄蕊的丢失过程。高自交率可能进一步强化由上位性互作与低遗传变异带来的性状丢失限制，这表明这类限制机制在自交物种中可能广泛存在。