Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee, 37996, U.S.A.
Am J Bot. 2019 Jul;106(7):1011-1020. doi: 10.1002/ajb2.1326. Epub 2019 Jul 11.
Male gametophytes of most seed plants deliver sperm to eggs via a pollen tube. Pollen tube growth rates (PTGRs) of angiosperms are exceptionally rapid, a pattern attributed to more effective haploid selection under stronger pollen competition. Paradoxically, whole genome duplication (WGD) has been common in angiosperms but rare in gymnosperms. Pollen tube polyploidy should initially accelerate PTGR because increased heterozygosity and gene dosage should increase metabolic rates. However, polyploidy should also independently increase tube cell size, causing more work which should decelerate growth. We asked how genome size changes have affected the evolution of seed plant PTGRs.
We assembled a phylogenetic tree of 451 species with known PTGRs. We then used comparative phylogenetic methods to detect effects of neo-polyploidy (within-genus origins), DNA content, and WGD history on PTGR, and correlated evolution of PTGR and DNA content.
Gymnosperms had significantly higher DNA content and slower PTGR optima than angiosperms, and their PTGR and DNA content were negatively correlated. For angiosperms, 89% of model weight favored Ornstein-Uhlenbeck models with a faster PTGR optimum for neo-polyploids, whereas PTGR and DNA content were not correlated. For within-genus and intraspecific-cytotype pairs, PTGRs of neo-polyploids < paleo-polyploids.
Genome size increases should negatively affect PTGR when genetic consequences of WGDs are minimized, as found in intra-specific autopolyploids (low heterosis) and gymnosperms (few WGDs). But in angiosperms, the higher PTGR optimum of neo-polyploids and non-negative PTGR-DNA content correlation suggest that recurrent WGDs have caused substantial PTGR evolution in a non-haploid state.
大多数种子植物的雄性配子体通过花粉管将精子输送到卵子。被子植物的花粉管生长速率（PTGR）非常快，这种模式归因于在更强的花粉竞争下，更有效的单倍体选择。矛盾的是，全基因组加倍（WGD）在被子植物中很常见，但在裸子植物中却很少见。花粉管多倍体最初应该会加速 PTGR，因为增加的异质性和基因剂量应该会增加代谢率。然而，多倍体也应该独立地增加管细胞的大小，从而导致更多的工作，从而减缓生长。我们想知道基因组大小的变化如何影响种子植物 PTGR 的进化。
我们组装了一个具有已知 PTGR 的 451 种植物的系统发育树。然后，我们使用比较系统发育的方法来检测新多倍体（属内起源）、DNA 含量和 WGD 历史对 PTGR 的影响，以及 PTGR 和 DNA 含量的相关性进化。
裸子植物的 DNA 含量明显高于被子植物，PTGR 最优值也较慢，而且它们的 PTGR 和 DNA 含量呈负相关。对于被子植物，89%的模型权重有利于具有更快 PTGR 最优值的新多倍体的 Ornstein-Uhlenbeck 模型，而 PTGR 和 DNA 含量之间没有相关性。对于属内和种内细胞型对，新多倍体的 PTGR < 古多倍体。
当 WGD 的遗传后果最小化时，如在种内同源多倍体（杂种优势低）和裸子植物（很少有 WGD）中，基因组大小的增加应该会对 PTGR 产生负面影响。但在被子植物中，新多倍体更高的 PTGR 最优值和非负的 PTGR-DNA 含量相关性表明，反复的 WGD 导致了非单倍体状态下的大量 PTGR 进化。
