Data from: Small and ugly? Phylogenetic analyses of the “selfing syndrome” reveal complex evolutionary fates of monomorphic primrose flowers

One of the most common trends in plant evolution, loss of self-incompatibility and ensuing increases in selfing, is generally assumed to be associated with a suite of phenotypic changes, notably a reduction of floral size, termed the selfing syndrome. We investigate whether floral morphological traits indeed decrease in a deterministic fashion after losses of self-incompatibility, as traditionally expected, using a phylogeny of 124 primrose species containing nine independent transitions from heterostyly (heteromorphic incompatibility) to homostyly (monomorphic self-compatibility), a classic system for evolution of selfing. We find similar overall variability of homostylous and heterostylous species, except for diminished herkogamy in homostyles. Bayesian mixed models demonstrate differences between homostylous and heterostylous species in all traits, but net effects across species are small (except herkogamy) and directionality differs among traits. Strongly drift-like evolutionary trajectories of corolla tube length and corolla diameter inferred by Ornstein-Uhlenbeck models contrast with expected deterministic trajectories toward small floral size. Lineage-specific population genetic effects associated with evolution of selfing may explain that reductions of floral size represent one of several possible outcomes of floral evolution after loss of heterostyly in primroses. Contrary to the traditional paradigm, selfing syndromes may, but do not necessarily evolve in response to increased selfing.植物进化中最常见的趋势之一——自交不亲和性丧失以及随之而来的自交率提升——通常被认为与一系列表型变化相关，其中尤以花大小缩减为代表，这类变化被称为自交综合征（selfing syndrome）。本研究以包含9次独立的花柱异长（异型自交不亲和）向花柱同长（同型自交亲和）转变事件的124种报春花属植物的系统发育树为研究材料，旨在验证传统认知中，自交不亲和性丧失后，花部形态性状是否确实会以确定性模式发生缩减——这一经典系统被广泛用于自交进化研究。研究结果显示，花柱同长物种与花柱异长物种的整体变异度相近，仅花柱同长物种的雌雄异位程度有所降低。贝叶斯混合模型分析表明，花柱同长与异长物种在所有花部性状上均存在显著差异，但跨物种的净效应普遍较小（雌雄异位性状除外），且不同性状的演化方向各不相同。通过奥恩斯坦-乌伦贝克（Ornstein-Uhlenbeck）模型推断得出的花冠管长度与花冠直径的类漂变演化轨迹，与学界预期的向小花方向演化的确定性轨迹相悖。与自交进化相关的谱系特异性群体遗传效应，或可解释为何在报春花属植物中，花大小缩减仅为花柱异长丧失后花部演化的多种可能结果之一。与传统认知范式相悖的是，自交综合征可能会随着自交率提升而演化，但并非必然如此。