Nehr, Z. | Search bioRxiv

bioRxiv2021

Tip growth in the brown alga Ectocarpus is controlled by a RHO-GAP-BAR domain protein independently from F-actin organisation

The brown alga Ectocarpus is a filamentous seaweed that grows by tip growth and branching. In the morphometric mutant etoile, tip growth is slower than in the WT and eventually stops. In this paper, we show that the causal etoile mutation is a null mutation in a bi-domain BAR-RhoGAP gene. By quantitative RT-PCR, we showed that ETOILE is ubiquitously expressed in prostrate filaments of the Ectocarpus sporophyte, and is downregulated in the etoile mutant. We immunolocalised both domains of the protein in WT and etoile, as well as RAC1, the known target of Rho-GAP enzymes. Thus, ETOILE would be localised at the apical cell dome where it would control the localisation of EsRAC1 to the plasma membrane. Actin staining showed that the mutant is not affected in F-actin structures. Overall, these results suggest that in Ectocarpus, BAR-RhoGAP controls tip growth by controlling RAC1 localization and through an actin-independent mechanism.

developmental biology↗

bioRxiv2021

SELECTION DRIVES THE EVOLUTION OF CONVERGENT GENE EXPRESSION CHANGES DURING TRANSITIONS TO CO-SEXUALITY IN HAPLOID SEXUAL SYSTEMS

AO_SCPLOWBSTRACTC_SCPLOWCo-sexuality has evolved repeatedly from ancestors with separate sexes across a wide range of taxa. The switch to co-sexuality is expected to involve major molecular readjustments at the level of gene expression patterns, as modified males or females will express the opposite sexual function for which their phenotypes have been optimized. However, the molecular changes underpinning this important transition remain unknown, particularly in organisms with haploid sexual systems such as bryophytes, red and brown algae. Here, we explore four independent events of emergence of co-sexuality from uni-sexual (dioicous) ancestors in brown algal clades in order to examine the nature, evolution and degree of convergence of gene expression changes that accompany the breakdown of dioicy. The amount of male versus female phenotypic differences in dioicous species were not correlated with the extent of sex-biased gene expression, in strike contrast to what is observed in animals. Although sex-biased genes exhibited a high turnover rate during brown alga diversification, their predicted functions were remarkably conserved. Transition to co-sexuality consistently involved adaptive gene expression shifts and rapid sequence evolution, particularly of male-biased genes. The gene expression profiles of co-sexual species were more similar to those of females than to males of related dioicous species, suggesting that the former may have arisen from ancestral females. Finally, we identified extensive convergent gene expression changes associated with the transition to co-sexuality, and these changes appear to be driven by selection. Together, our observations provide novel insights on how co-sexual systems arise from ancestral, haploid UV sexual systems.

evolutionary biology↗