Integrative systematics of the Brazilian tree “sucupira”: morphological, cytogenetic and genomic boundaries of <i>Pterodon emarginatus</i> Vogel and <i>P. pubescens</i> (Benth.) Benth. (Leguminosae, Papilionoideae)

In the genus <i>Pterodon</i> (Leguminosae, Papilionoideae) <i>P. emarginatus</i> and <i>P. pubescens</i> (both known as “sucupira”) exhibit unclear boundaries due to their incomplete isolation barriers and phylogenetic proximity. Both species have a conserved karyotype with 2<i>n</i> = 16, featuring small and morphologically similar chromosomes, and they form hybrids in sympatric areas of distribution in the Neotropical Savanna. Here, we aimed to revisit the systematics of <i>P. emarginatus</i> and <i>P. pubescens</i> based on morphometry of vegetative and reproductive characters, cytogenetics and genome skimming (low coverage Illumina sequencing). The morphometric analysis based on continuous data resulted in unclear distinctiveness between <i>P. emarginatus</i> and <i>P. pubescens</i>. However, the discrete characters of pubescence on leaves and flowers allow us to distinguish these species and individuals with intermediate morphology. The species presented similar genome sizes (1 C = ca. 0.64 pg) and stable karyotypes with two pairs of CMA⁺/DAPI⁻ bands/35S rDNA and one pair of 5S rDNA sites. Similarities were also observed in the repetitive genomic fraction (∼25%) with few differences in the retrotransposons (RTs) abundances. The most representative RTs in both species were Ty1/copia-SIRE, Ty3/gypsy-Athila and Ty3/gypsy-Tekay. Remarkably, we found a difference in the 5S rDNA cluster, with the nontranscribed spacer (NTS) regions varying in length and similarity between <i>P. emarginatus</i> and <i>P. pubescens</i>. Six satellite DNAs were found, two of which were exclusive to <i>P. emarginatus</i> and four were shared by both species. <i>In situ</i> hybridization revealed that the most abundant repeats were enriched in proximal CMA⁺ heterochromatin in both species, except for the Ty1/copia-Ale retroelement, which was widely dispersed across all <i>P. pubescens</i> chromosomes. Our morphological, cytogenetic and genomic data revealed that <i>Pterodon</i> species boundaries are complex corroborating the evolutionary proximity between these species. We suggest that the cytogenetic and genomic stability found in <i>Pterodon</i> may facilitate the natural interspecific gene flow reported in sympatric populations, complicating the interpretation of morphological traits and the understanding of their systematics and evolution.

在豆科（Leguminosae）蝶形花亚科（Papilionoideae）翼豆属（Pterodon）中，凹叶翼豆（P. emarginatus）与毛翼豆（P. pubescens，二者均被称为“sucupira”）由于生殖隔离屏障不完善且系统发育亲缘关系较近，分类界限模糊不清。两个物种均具有保守核型，染色体数目为2n=16，染色体体型小巧且形态相似；在新热带稀树草原的同域分布区域内，二者可形成杂交种。本研究基于营养与繁殖性状的形态测量学（morphometry）、细胞遗传学（cytogenetics）以及基因组浅层测序（genome skimming，低覆盖度Illumina测序），重新审视凹叶翼豆与毛翼豆的分类学问题。基于连续性状的形态测量分析未能清晰区分凹叶翼豆与毛翼豆，但叶片与花部的被毛离散性状可用于区分二者以及具有中间形态的个体。两个物种的基因组大小相近（1C≈0.64 pg），核型稳定，均具有2对CMA+/DAPI-位点/35S核糖体DNA（rDNA）以及1对5S rDNA位点。二者的重复基因组组分占比相似（约25%），仅在反转录转座子（retrotransposons, RTs）的丰度上存在少量差异。两个物种中最具代表性的反转录转座子为Ty1/copia-SIRE、Ty3/gypsy-Athila以及Ty3/gypsy-Tekay。值得注意的是，二者的5S rDNA基因簇存在差异，凹叶翼豆与毛翼豆的非转录间隔区（nontranscribed spacer, NTS）长度与序列相似性均有所不同。本研究共发现6种卫星DNA（satellite DNAs），其中2种为凹叶翼豆所特有，剩余4种为二者共有。原位杂交（in situ hybridization）结果显示，除Ty1/copia-Ale反转录元件外，两个物种中丰度最高的重复序列均富集于近端CMA+异染色质（heterochromatin）区域，而Ty1/copia-Ale反转录元件则广泛分布于毛翼豆的所有染色体上。本研究的形态学、细胞遗传学与基因组学数据表明，翼豆属物种的分类界限较为复杂，印证了二者之间较近的进化亲缘关系。我们推测，翼豆属所具备的细胞遗传学与基因组学稳定性，可能促进了同域种群中已报道的自然种间基因流（interspecific gene flow），进而使得形态性状的解析以及该类群分类与进化的研究更为复杂。