Rapid turnover of life-cycle-related genes in the brown algae

Abstract

BackgroundSexual life cycles in eukaryotes involve a cyclic alternation between haploid and diploid phases. While most animals possess a diploid life cycle, plants and algae alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations. In many algae, gametophytes and sporophytes are independent and free living, and may present dramatic phenotypic differences. The same shared genome can therefore be subject to different, even conflicting, selection pressures in each of the life cycle generations. Here, we have analysed the nature and extent of genome-wide generation-biased gene expression in four species of brown algae with contrasting levels of dimorphism between life cycle generations, in order to assess the potential role of generation-specific selection in shaping patterns of gene expression and divergence.\n\nResultsWe show that the proportion of the transcriptome that is generation-biased is associated with the level of phenotypic dimorphism between the life cycle stages. Importantly, our data reveals a remarkably high turnover rate for life-cycle-related gene sets across the brown algae and highlights the importance not only of co-option of regulatory programs from one generation to the other but also a key role for newly emerged, lineage-specific genes in the evolution of the gametophyte and sporophyte developmental programs in this major eukaryotic group. Moreover, we show that generation-biased genes display distinct evolutionary modes, with gametophyte-biased genes evolving rapidly at the coding sequence level whereas sporophyte-biased genes exhibit changes in their patterns of expression.\n\nConclusionOur analysis uncovers the characteristics, expression patterns and evolution of generation-biased genes and underline the selective forces that shape this previously underappreciated source of phenotypic diversity.