Evolution and morphological diversity of the genus Diploneis (Bacillariophyta) in Lake Malawi, with description of six new species

Lake Malawi, one of the world’s oldest and most important freshwater systems, harbours a remarkable biotic diversity partly due to variable fluctuations in climate and lake level. Over time, these fluctuations have contributed to the emergence and development of different ecological niches and microhabitats that have facilitated diversification within the lake. Despite our extensive knowledge of different animal groups, the evolutionary history of species-rich diatom groups in Africa and, particularly, in Lake Malawi has yet to be studied. Here, we used a combination of morphological, molecular, and paleontological data to assess the species richness and evolutionary processes of the diatom genus Diploneis in this lake. The comprehensive dataset revealed four putative new endemic species from modern and two apparently extinct Diploneis species from the fossil record of the lake, described here as new to science. Morphological analyses showed that all these new species exhibit remarkable differences and unique morphological patterns that were recently discovered in Diploneis species from nearby Lake Tanganyika. Molecular phylogenetic analyses revealed that the four new species now living in Lake Malawi are nested within a clade of Lake Tanganyika Diploneis species but do not form a monophyletic group. This suggests that the new Lake Malawi Diploneis species are derived from one or more Tanganyikan ancestors and that more than one colonization event probably occurred. However, further analyses are needed to elucidate their origin.