Togashi Tatsuya, Horinouchi Yusuke, Parker Geoff A
Marine Biosystems Research Center, Chiba University, Kamogawa 299-5502, Japan.
Department of Evolution, Ecology and Behaviour, University of Liverpool, Liverpool L69 7ZB, UK.
R Soc Open Sci. 2021 Mar 3;8(3):201611. doi: 10.1098/rsos.201611.
Gamete dynamics theory proposes that anisogamy arises by disruptive selection for gamete numbers versus gamete size and predicts that female/male gamete size (anisogamy ratio) increases with adult size and complexity. Evidence has been that in volvocine green algae, the anisogamy ratio correlates positively with haploid colony size. However, green algae show notable exceptions. We focus on Bryopsidales green algae. While some taxa have a diplontic life cycle in which a diploid adult (=fully grown) stage arises directly from the zygote, many taxa have a haplodiplontic life cycle in which haploid adults develop indirectly: the zygote first develops into a diploid adult (sporophyte) which later undergoes meiosis and releases zoospores, each growing into a haploid adult gametophyte. Our comparative analyses suggest that, as theory predicts: (i) male gametes are minimized, (ii) female gamete sizes vary, probably optimized by number versus survival as zygotes, and (iii) the anisogamy ratio correlates positively with diploid (but not haploid) stage complexity. However, there was no correlation between the anisogamy ratio and diploid adult stage size. Increased environmental severity (water depth) appears to drive increased diploid adult stage complexity and anisogamy ratio: gamete dynamics theory correctly predicts that anisogamy evolves with the (diploid) stage directly provisioned by the zygote.
配子动力学理论提出，异形配子的产生是由于对配子数量与配子大小的分裂选择，并预测雌性/雄性配子大小（异形配子比例）会随着成年个体大小和复杂性的增加而增加。有证据表明，在团藻目绿藻中，异形配子比例与单倍体群体大小呈正相关。然而，绿藻也有显著的例外情况。我们聚焦于羽藻目绿藻。虽然一些分类群具有二倍体生命周期，其中二倍体成年（=完全成熟）阶段直接由合子产生，但许多分类群具有单倍体-二倍体生命周期，其中单倍体成年个体间接发育：合子首先发育成二倍体成年个体（孢子体），随后进行减数分裂并释放游动孢子，每个游动孢子发育成一个单倍体成年配子体。我们的比较分析表明，正如理论所预测的那样：（i）雄配子最小化，（ii）雌配子大小各异，可能是通过作为合子的数量与存活率之间的优化来实现的，并且（iii）异形配子比例与二倍体（而非单倍体）阶段的复杂性呈正相关。然而，异形配子比例与二倍体成年阶段大小之间没有相关性。环境严峻程度的增加（水深）似乎推动了二倍体成年阶段复杂性和异形配子比例的增加：配子动力学理论正确地预测了异形配子是随着合子直接提供的（二倍体）阶段而进化的。
