Data from: Invasion success in polyploids: the role of inbreeding in the contrasting colonization abilities of diploid versus tetraploid populations of Centaurea stoebe s.l

As a consequence of founder effects, inbreeding can hamper colonization success: First, in species with self-incompatibility controlled by an S-locus, inbreeding may decrease cross-compatibility, mainly due to the sharing of identical S-alleles between closely related mating partners. Secondly, inbreeding can reduce fitness of inbred relative to outbred offspring (i.e. inbreeding depression). Polyploids often show reduced inbreeding depression compared to diploids, which may contribute to the overrepresentation of polyploids among invasive species. This is the first study that tests how the effects of inbreeding differ between geocytotypes (i.e. ploidy levels within a given range).
Our model organism, Centaurea stoebe, is strictly self-incompatible and comprises three geocytotypes: diploids are more frequent than tetraploids in the native range, while only tetraploids occur in the invasive range. We conducted a breeding experiment (sib-mating vs. outcrossing) with 14 native diploid, 13 native tetraploid and 15 invasive tetraploid populations. We recorded cross-compatibility and estimated a cumulative index for offspring fitness. Since frequent inbreeding can result in purging of genetic load responsible for inbreeding depression, our analyses included a metric for within-population relatedness, based on eight microsatellite markers, to assess the effect of purging.
Inbreeding was found to reduce cross-compatibility, which was similarly pronounced in diploids and tetraploids. It also caused inbreeding depression in cumulative fitness, which was significant in diploids but not in tetraploids. No evidence of purging was observed as inbred fitness was not affected by within-population relatedness.
Synthesis. Our results provide new insights into the contrasting invasion success of the cytotypes of C. stoebe. As the effects of cross-compatibility and purging were comparable between cytotypes, both processes can be ruled out to affect the colonization success of diploids versus tetraploids. Our findings are consistent with the hypothesis that polyploidy increases the masking of recessive mutations, which maintains high fitness in inbred tetraploids and may thus facilitate colonization of new ranges. We highlight that reduced inbreeding depression may add to previously acknowledged advantages of polyploids in range expansions, a mechanism that may hitherto have been underestimated due to a lack of data on variation in inbreeding depression across geocytotypes.

奠基者效应（founder effects）的后果之一是近交作用会阻碍定殖成功：其一，对于由S位点（S-locus）调控的自交不亲和性（self-incompatibility）物种而言，近交可能会降低杂交亲和性，这主要是因为亲缘关系较近的交配双方共享了相同的S等位基因（S-alleles）。其二，近交可降低近交后代相较于远交后代的适合度，即近交衰退（inbreeding depression）。与二倍体（diploids）相比，多倍体（polyploids）通常表现出更弱的近交衰退，这或许可以解释入侵物种中多倍体占比偏高的现象。本研究首次探讨了近交的效应在地理倍型（geocytotypes，即特定分布范围内的不同倍性水平）之间的差异。

本研究的模式生物为矢车菊（Centaurea stoebe），该物种严格自交不亲和，且包含三种地理倍型：在原生分布区中二倍体的占比高于四倍体（tetraploids），而入侵分布区中仅存在四倍体。我们开展了一项交配实验（同胞交配vs.远交），实验材料涵盖14个原生二倍体种群、13个原生四倍体种群以及15个入侵四倍体种群。我们记录了杂交亲和性，并估算了后代适合度的累积指数。鉴于频繁的近交可清除引发近交衰退的遗传负荷，我们的分析纳入了基于8个微卫星标记（microsatellite markers）的种群内亲缘关系指标，以评估遗传清除效应。

研究发现，近交可降低杂交亲和性，且该效应在二倍体和四倍体中表现程度相近。近交还会导致累积适合度出现近交衰退，该现象在二倍体中显著，但在四倍体中并不明显。由于近交后代的适合度并未受到种群内亲缘关系的影响，因此未观察到遗传清除的证据。

综合与讨论。本研究结果为矢车菊不同倍型之间入侵成功的差异提供了新的见解。由于不同倍型的杂交亲和性效应与遗传清除效应均无显著差异，因此可排除这两个过程对二倍体与四倍体定殖成功的影响。我们的研究结果与“多倍体可增强隐性突变的掩盖效应”这一假说相符，该效应可维持近交四倍体的高适合度，从而助力其拓展新的分布范围。本研究强调，近交衰退的减弱或许可补充多倍体在范围扩张中已被证实的优势，而此前由于缺乏地理倍型间近交衰退变异的数据，该机制的重要性一度被低估。