Current tree diversity in temperate areas of the Northern Hemisphere is characterized by disjunct hotspots in North America and East Asia. It remains debated on the causes of this pattern. The Boreotropical hypothesis proposes that a widespread boreotropical flora developed at midlatitudes of the Northern Hemisphere during the early Paleogene and spread around the globe via the Beringia and North Atlantic land bridges and shores of the Tethys Seaway.
In a study published in Systematic Biology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators from Switzerland used the walnut family Juglandaceae as a case study to evaluate different hypotheses about the origin of intercontinental disparities in temperate woody plant diversity.
By using the exceptionally rich fossil record of the walnut family, the researchers inferred a phylogenetic tree of extinct and extant species and used it to infer their history of dispersal, extinction, and niche evolution.
They compiled a comprehensive data set of fruit macrofossil records of Juglandaceae based on the Cenozoic Angiosperm Database and published literature. They also developed a new model of trait evolution based on the fossilized Brownian motion model implemented in fossilBM.
"We integrated molecular, morphological, and (paleo)environmental data in a comprehensive analytical framework, including a new Bayesian model for inferring time-variable trends in climatic niche evolution," said ZHANG Qiuyue, first author of the study.
The researchers found that the sister clades Engelhardioideae and Juglandoideae originated in a semiarid to humid, warm environment but then diverged in their evolutionary trajectories. The Engelhardioideae gradually adapted to wetter tropical climates and Juglandoideae expanded across temperate regions with increasingly variable and seasonal climate.
"With the inclusion of extinct lineages, we find robust support for a Boreotropical origin of Juglandaceae and contrasting evolutionary trajectories in its subfamilies. It shows how Cenozoic climate change played a key role in modulating dispersal, local extinctions and adaptation to changing environments," said Prof. XING Yaowu, principal investigator of the study.
