Fernando Tuya Cortés, from ECOAQUA’s Biodiversity and Conservation Group, discusses his article: ‘Strong phylogenetic signal and models of trait evolution evidence phylogenetic niche conservatism for seagrasses.’
Aquatic flowering plants
Seagrasses are a group of marine angiosperms (i.e., flowering plants) fully adapted to a submerged life. They are found thoughout the world’s oceans, distributed from tropical to polar coastal areas around the globe. Around 72 species of seagrass create habitats of paramount ecological, socio-cultural, and economic value. These habitats are of considerable conservation interest and facing a range of human-induced stressors. Previous studies of seagrass diversity have mostly focused on contemporary, species-level, metrics, that otherwise ignore the phylogenetic relationships of seagrasses, despite their potential evolution from common ancestors.
Phylogenetic Signal (PS) is the tendency of phylogenetically related (‘sibling’) species to resemble each other, in terms of a range of traits, such as their morphology. Signatures of PS, coupled with models of niche evolution (i.e., how ‘fast’ traits have evolved), can help to detect Phylogenetic Niche Conservatism (PNC). According to PNC, close relatives (‘sibling’ species) live in comparable niches, so there is a tendency of species to retain ancestral ecological traits.
The goal of our study was to assess the pattern of PS for the world’s seagrasses, by testing the non-independence of phylogenetic relatedness and seagrass species traits. A phylogeny of 49 seagrass species was constructed, together with a matrix of 9 traits covering morphological, life-history and reproductive attributes. This represented 70% of the world’s seagrasses.
Our results supported the existence of strong PS for seagrasses, with reproductive traits in particular being similar between closely related species. This follows an ‘Early Burst’ evolution model. In this model, large initial phenotypic divergence follows the adaptation of a group to exploit a new environment. This leads to rapid rates of evolution at the root of the phylogenetic tree. Because seagrasses are aquatic plants adapted to marine environments that evolved from terrestrial ancestors, our results demonstrate that seagrasses experienced notable initial adaptations, at least for their reproductive traits, to thrive in a high-salinity domain. In other words, ancient seagrasses faced severe initial selective pressures through a range of physiological, morphological and, importantly, reproductive modifications, relative to their terrestrial congeners.
Implications for seagrass evolution
The pattern of strong PS across the sea grasses seems to be a consequence of long-term PNC of seagrass traits after initial radiation. Therefore, we provide support for the relevance of evolution from common seagrass ancestors that initially adapted to the marine environment, without major subsequent bottlenecks. This supposes that most evolution of studied traits has been hampered by phylogenetic inertia, which seems a stronger driver of seagrass traits rather than recent environmental processes. Without a doubt, improvements in seagrass phylogenetic relationships through modern genomic tools and multi-traits databases will encourage researchers to revisit connections between functional (ecological) and phylogenetic seagrass similarities in the promising field of seagrass macroevolution.