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Johannes BergstenForskare, Docent


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  • Phylogeny, distribution and speciation in the endemic diving beetle genus Pachynectes on Madagascar

    Rasa Bukontaite (et al.).

    Madagascar is well known for high levels of endemism in both the flora and fauna. More specifically most of Madagascars' endemic species have extremely restricted regional distributions, which is a characteristic of this large island’s biodiversity. Biologists are still struggling to understand the speciation processes that have produced this pattern of diversity. Several general hypotheses have been developed trying to explain the origin and microendemism of Malagasy species. Two main hypotheses focus on watersheds and climate gradients. Here, we use a phylogeny in combination with data on spatial distribution of the endemic diving beetle genus Pachynectes (Coleoptera: Dytiscidae: Bidessini) to critically evaluate the watershed versus the climatic gradient hypotheses of this river dwelling insect radiation. Based on field sampling from expeditions between 2006 and 2014, we first show that the species diversity in this genus is at least three times higher than previously believed. We reconstructed a molecular phylogeny based on five gene fragments and recovered three main clades: a western clade, an eastern clade and a "keeled" clade diagnosable based on an elytral keel. The western and keeled clades both occur mainly in the drier western parts of the country, while species in the eastern clade are typical of rivers in the eastern humid rainforest. Notably, sister species were always allopatrically distributed along either latitudinal or longitudinal-altitudinal gradients. Based on species distribution modelling (SDM) and our phylogenetic data, we conclude that the microendemic patterns of Pachynectes species are unlikely to be explained by the watershed hypothesis. In contrast, the four main bioclimatic regions are more likely to have been causally involved in the past speciation processes and might explain the current distribution pattern in this insect genus.  

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