A vast diversity of types of life cycles exists in nature, and several theories have been advanced to explain how this diversity has evolved and how each type of life cycle is retained over evolutionary time. Here, we exploited the diversity of life cycles and reproductive traits of the brown algae (Phaeophyceae) to test several hypotheses on the evolution of life cycles. We investigated the evolutionary dynamics of four life-history traits: life cycle, sexual system, level of gamete dimorphism and gamete parthenogenetic capacity. We assigned states to up to 77 representative species of the taxonomic diversity of the brown algal group, in a multi-gene phylogeny. We used maximum likelihood and Bayesian analyses of correlated evolution, while taking the phylogeny into account, to test for correlations between traits and to investigate the chronological sequence of trait acquisition. Our analyses are consistent with the prediction that diploid growth evolves when sexual reproduction is preferred over asexual reproduction, possibly because it allows the complementation of deleterious mutations. We also found that haploid sex determination is ancestral in relation to diploid sex determination. However, our results could not address whether increased zygotic and diploid growth are associated with increased sexual dimorphism. Our analyses suggest that in the brown algae, isogamous species evolved from anisogamous ancestors, contrary to the commonly reported pattern where evolution proceeds from isogamy to anisogamy.

中文翻译：

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生命周期和繁殖特征的进化：来自褐藻的见解。


自然界存在着多种多样的生命周期类型，并且已经提出了几种理论来解释这种多样性是如何进化的以及每种类型的生命周期如何在进化过程中保留下来。在这里，我们利用褐藻（褐藻纲）生命周期和繁殖特征的多样性来测试关于生命周期进化的几种假设。我们研究了四种生活史特征的进化动力学：生命周期、性系统、配子二态性水平和配子孤雌生殖能力。在多基因系统发育中，我们将褐藻类分类多样性的多达 77 个代表性物种分配为状态。我们使用相关进化的最大似然和贝叶斯分析，同时考虑系统发育，来测试性状之间的相关性并研究性状获得的时间顺序。我们的分析与预测一致，即当有性繁殖优于无性繁殖时，二倍体生长就会进化，可能是因为它允许有害突变的互补。我们还发现单倍体性别决定是与二倍体性别决定相关的祖先。然而，我们的结果无法解决合子和二倍体生长的增加是否与性二态性的增加有关。我们的分析表明，在褐藻中，同配物种是从异配祖先进化而来的，这与通常报道的从同配到异配的进化模式相反。