Department of Biology, San Francisco State University, San Francisco, California, 94132, USA.
San Francisco Bay National Estuarine Research Reserve, Estuary & Ocean Science Center, San Francisco State University, Romberg Tiburon Campus, 3150 Paradise Drive, Tiburon, California, 94920, USA.
Am J Bot. 2020 Dec;107(12):1798-1814. doi: 10.1002/ajb2.1576. Epub 2020 Dec 3.
Hybridization plays a key role in introgressive adaptation, speciation, and adaptive radiation as a source of evolutionary innovation. Hybridization is considered common in Arctostaphylos, yet species boundaries are retained in stands containing multiple species. Arctostaphylos contains diploids and tetraploids, and recent phylogenies indicate two clades; we hypothesize combinations of these traits limit or promote hybridization rates.
We statistically analyzed co-occurrence patterns of species by clade membership and ploidy level from 87 random 0.1 ha plots. We sampled multiple sites to analyze for percent hybridization based on morphology. Finally, phenophases were analyzed by scoring herbarium sheets for a large number of taxa from both clades as well as tetraploids, and second, surveying three field sites over two years for divergence in phenological stages between co-occurring taxa.
Most taxa in Arctostaphylos are allopatric relative to other congenerics. When two taxa co-occur, the patterns are a diploid with a tetraploid, or two diploids from different clades. When three taxa co-occur, the pattern is two diploids from different clades and a tetraploid. Field and herbarium data both indicate flowering phenology is displaced between diploids from the two clades; one of the diploid clades and tetraploids overlap considerably.
The two deep clades in Arctostaphylos are genetically distant, with hybrids rare or non-existent when taxa co-occur. Reproductive isolation between clades is enhanced by displaced flowering phenology for co-occurring species. Within clades, taxa appear to have few reproductive barriers other than an allopatric distribution or different ploidy levels.
杂交在渐渗适应、物种形成和适应性辐射中起着关键作用，是进化创新的来源。杂交在熊果酸中被认为很常见，但在包含多个物种的种群中保留了物种边界。熊果酸包含二倍体和四倍体，最近的系统发育表明有两个分支；我们假设这些特征的组合限制或促进了杂交率。
我们通过 87 个随机 0.1 公顷的样方，统计分析了物种的共现模式，这些样方按分支成员和倍性水平划分。我们在多个地点取样，根据形态学分析杂交百分率。最后，我们通过对两个分支的大量分类群以及四倍体的标本进行评分，分析了物候期；其次，在两年内对三个野外地点进行了调查，以研究共存分类群之间在物候阶段上的差异。
熊果酸中的大多数分类群与其他同属种都是异地的。当两个分类群共存时，模式是一个二倍体和一个四倍体，或者是来自不同分支的两个二倍体。当三个分类群共存时，模式是两个来自不同分支的二倍体和一个四倍体。野外和标本数据都表明，两个分支的二倍体的开花物候期存在差异；其中一个二倍体分支和四倍体有很大的重叠。
熊果酸的两个深分支在遗传上是遥远的，当分类群共存时，杂交很少或不存在。分支间的生殖隔离因共存物种的开花物候期错位而增强。在分支内，除了分布的异地或不同的倍性水平外，分类群之间似乎没有多少生殖障碍。
