Abstract
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.
Highlights
The life cycle of an organism is one of its most fundamental features and influences the evolution of a variety of traits, including mode of reproduction, developmental processes, mode of dispersal, adaptation to local environment and ecological success
We tested additional hypotheses, including the possibility that gamete size influences the capacity for asexual reproduction through parthenogenesis (Luthringer et al, 2014), and we discuss the macro-evolutionary dynamics of transitions between sexual systems in the brown algae
Our analysis suggests that oogamy is most likely the ancestral state in the brown algae (Table S2, Figure 2E-F)
Summary
The life cycle of an organism is one of its most fundamental features and influences the evolution of a variety of traits, including mode of reproduction, developmental processes, mode of dispersal, adaptation to local environment and ecological success. One important feature of sexual life cycles in eukaryotes is the degree of similarity between male and female gametes This ‘gamete dimorphism’ is a continuous trait, and a number of models have been proposed to explain how anisogamous organisms could evolve from an isogamous ancestor (Hoekstra, 1980; Randerson & Hurst, 2001). Testing evolutionary hypotheses regarding the causes and consequences of life history trait diversity requires data from multiple species placed in a phylogenetic context Such comparative studies have been hampered by a lack of accessible data regarding life cycles, sexual systems and sex determination mechanisms across the eukaryotic tree of life, and most in groups outside animals and land plants. We tested additional hypotheses, including the possibility that gamete size influences the capacity for asexual reproduction through parthenogenesis (Luthringer et al, 2014), and we discuss the macro-evolutionary dynamics of transitions between sexual systems in the brown algae
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