Global biogeography and diversification of a group of brown seaweeds (Phaeophyceae) driven by clade-specific evolutionary processes

Aim: Historical processes that shaped current diversity patterns of
seaweeds remain poorly understood. Using Dictyotales, a globally
distributed order of brown seaweeds as a model, we test if historical
biogeographical and diversification patterns are comparable across clades.
Dictyotales contain some 22 genera, three of which, Dictyota, Lobophora
and Padina, are exceptionally diverse. Specifically we test if the
evolutionary processes in these clades that shaped their latitudinal
diversity patterns are in line with the tropical conservatism, the
out-of-the-tropics, and diversification rate hypotheses. Location: Global
coastal benthic marine environments. Taxon: Dictyotales (Phaeophyceae)
Methods: Species diversity was inferred using DNA-based species
delineation, addressing cryptic diversity and circumventing taxonomic
problems. A six-gene time-calibrated phylogeny, distribution data of 3,755
specimens, and probabilistic modeling of geographic range evolution was
used to infer historical biogeographical patterns. The phylogeny was
tested against different trait dependent models in order to compare
diversification rates for different geographical units as well as
different thermal affinities. Results: The Dictyotales originated in the
Middle Jurassic and reach a current peak of species diversity in the
Central Indo-Pacific. Ancestral range estimation points to a southern
hemisphere origin of Dictyotales corresponding to the tropical southern
Tethys Sea. Our results demonstrate that diversification rates were
generally higher in tropical regions, but increased diversification rates
in different clades are driven by different processes. Three major clades
underwent a major diversification burst in the early Cenozoic, with
Dictyota and Padina expanding their distribution into temperate regions,
while Lobophora retained a predominantly tropical niche. Main conclusions:
Our results are consistent with both the tropical conservatism hypothesis,
in which clades originate and remain in the tropics (Lobophora), and the
out-of-the-tropics scenario, where taxa originate and expand towards the
temperate regions while preserving their presence in the tropics
(Dictyota, Padina).