Evolution and maintenance of haploid-diploid life cycles in natural populations: The case of the marine brown algaEctocarpus

Abstract

The evolutionary stability of haploid-diploid life cycles is still controversial. Mathematical models indicate that niche differences between ploidy phases may be a necessary condition for the evolution and maintenance of these life cycles. Nevertheless, experimental support for this prediction remains elusive. In the present work, we explored this hypothesis in natural populations of the brown alga Ectocarpus. Consistent with the life cycle described in culture, Ectocarpus crouaniorum in NW France and E. siliculosus in SW Italy exhibited an alternation between haploid gametophytes and diploid sporophytes. Our field data invalidated, however, the long-standing view of an isomorphic alternation of generations. Gametophytes and sporophytes displayed marked differences in size and, conforming to theoretical predictions, occupied different spatiotemporal niches. Gametophytes were found almost exclusively on the alga Scytosiphon lomentaria during spring whereas sporophytes were present year-round on abiotic substrata. Paradoxically, E. siliculosus in NW France exhibited similar habitat usage despite the absence of alternation of ploidy phases. Diploid sporophytes grew both epilithically and epiphytically, and this mainly asexual population gained the same ecological advantage postulated for haploid-diploid populations. Consequently, an ecological interpretation of the niche differences between haploid and diploid individuals does not seem to satisfactorily explain the evolution of the Ectocarpus life cycle.

High-density genetic map and identification of QTLs for responses to temperature and salinity stresses in the model brown alga Ectocarpus

“…A recent study in the Ectocarpus group has shown that haploid and diploid organisms also exhibit clear niche partitioning. However, mainly asexual populations also exhibit similar habitat usage25. Although brown algae are mainly marine species, rare cases of adaptation to fresh water have been reported26.…”

Parthenogenesis is rare in the reproduction of a sexual field population of the isogamous brown alga Scytosiphon (Scytosiphonaceae, Ectocarpales)

“…, , Couceiro et al. ). Several causes can be considered for explaining the rare occurrence of parthenogenesis.…”

“…High fertilization success (i.e., few unfused gametes) has been proposed as the cause of the rare parthenosporophytes (Peters and Müller , Couceiro et al. ). It is well known that brown algae have various mechanisms that enhance the probability of gamete encounter, such as synchronous gamete release, phototaxis, and attraction of male gametes by sexual pheromones (Brawley and Johnson ).…”

“…However, no parthenosporophytes were detected among nearly 200 field sporophytic thalli examined (Couceiro et al. ). Therefore, we hypothesized that parthenogenesis rarely contributes to reproduction in sexual populations, even if it can be observed under culture conditions.…”

The Ectocarpus IMMEDIATE UPRIGHT gene encodes a member of a novel family of cysteine-rich proteins with an unusual distribution across the eukaryotes

“…Recent work on the life cycles of Ectocarpus species under field conditions suggests that the sporophyte generation tends to persist for longer and to be the stage of the life cycle that allows the species to overwinter (Couceiro et al, 2015). The gametophyte generation, by contrast, tends to be ephemeral during the spring months and to have principally a reproductive role.…”