Department of Botany and Zoology, Masaryk University, Kotlářská, Brno, Czech Republic.
Bolus Herbarium, Department of Biological Sciences, University of Cape Town, Rondebosch, Cape Town, South Africa.
Ann Bot. 2020 Jul 24;126(2):323-330. doi: 10.1093/aob/mcaa095.
The idea that genome (size) evolution in eukaryotes could be driven by environmental factors is still vigorously debated. In extant plants, genome size correlates positively with stomatal size, leading to the idea that conditions enabling the existence of large stomata in fossil plants also supported growth of their genome size. We test this inductive assumption in drought-adapted, prostrate-leaved Cape (South Africa) geophytes where, compared with their upright-leaved geophytic ancestors, stomata develop in a favourably humid microclimate formed underneath their leaves.
Stomatal parameters (leaf cuticle imprints) and genome size (flow cytometry) were measured in 16 closely related geophytic species pairs from seven plant families. In each pair, representing a different genus, we contrasted a prostrate-leaved species with its upright-leaved phylogenetic relative, the latter whose stomata are exposed to the ambient arid climate.
Except for one, all prostrate-leaves species had larger stomata, and in 13 of 16 pairs they also had larger genomes than their upright-leaved relatives. Stomatal density and theoretical maximum conductance were less in prostrate-leaved species with small guard cells (<1 pL) but showed no systematic difference in species pairs with larger guard cells (>1 pL). Giant stomata were observed in the prostrate-leaved Satyrium bicorne (89-137 µm long), despite its relatively small genome (2C = 9 Gbp).
Our results imply that climate, through selection on stomatal size, might be able to drive genome size evolution in plants. The data support the idea that plants from 'greenhouse' geological periods with large stomata might have generally had larger genome sizes when compared with extant plants, though this might not have been solely due to higher atmospheric CO2 in these periods but could also have been due to humid conditions prevailing at fossil deposit sites.
真核生物基因组（大小）进化可能受环境因素驱动的观点仍在激烈争论中。在现存的植物中，基因组大小与气孔大小呈正相关，这导致了这样一种观点，即在化石植物中存在大气孔的条件也支持了它们基因组大小的增长。我们在适应干旱的、平卧叶的海角（南非）球根植物中检验了这个归纳假设，与它们直立叶的球根植物祖先相比，平卧叶植物的气孔在其叶片下方形成的有利湿润小气候中发育。
我们测量了来自 7 个科的 16 对密切相关的球根植物种的气孔参数（叶片角质层印痕）和基因组大小（流式细胞术）。在每一对中，代表一个不同的属，我们对比了一个平卧叶种与其直立叶的系统发育近亲，后者的气孔暴露在周围的干旱气候中。
除了一个种之外，所有的平卧叶种都具有更大的气孔，在 16 对中的 13 对中，它们的基因组也比直立叶的近亲大。在具有小保卫细胞（<1 pL）的平卧叶物种中，气孔密度和理论最大导度较低，但在具有较大保卫细胞（>1 pL）的物种对中没有系统差异。尽管 Satyrium bicorne（89-137 µm 长）的气孔较大，但它的基因组相对较小（2C = 9 Gbp）。
我们的结果表明，气候通过对气孔大小的选择，可能能够驱动植物的基因组大小进化。这些数据支持了这样一种观点，即在具有大气孔的“温室”地质时期的植物，与现存植物相比，其基因组大小通常更大，尽管这可能不仅仅是由于这些时期大气中 CO2 较高，还可能是由于化石沉积地点普遍存在潮湿条件。
