Palaeobiogeographic evolution and climatic adaptation within the plant family Linaceae

Around 37% of the Mediterranean territory is covered with polyploid angiosperms, of which the genus Linum occupies a considerable region. Linum is the most diversified genus of the plant family Linaceae that comprises a “family pair” of Hugonioideae distributed in tropical regions and Linoideae mainly distributed in temperate to subtropical regions. We provide the oldest fossils of Linaceae, comparable to the living genera Linum and Reinwardtia, from the Late Cretaceous Infratrappean Deccan volcano sedimentary sequences of Maharashtra, India. The phylogenetic analysis conducted by combining the morphological characters of the pollen fossils recovered and the pollen morphological as well as molecular characters of extant species of Linaceae reveals that the family originated in the wet and warm tropical zones of South America in the Late Cretaceous. Thereafter, the family dispersed to Africa giving rise to the lineage of Linoideae that further diverged into two main clades, one of which evolved either on seasonally wet areas of Kohistan-Ladakh Island arc (KLIA) or on the Indian Plate in Maastrichtian-Paleocene. The second clade encompassing the genus Linum originated on seasonally wet areas of Africa and later dispersed to Mediterranean region via Boreotropical route in Paleocene where it adapted to seasonally dry climatic conditions that prevailed during late Eocene. The family further expanded its geographical range and spread to Eurasia, and to North America via Bering land bridge. We also propose the dual colonization of Linaceae in India. The seasonally wet lineages dispersed from Africa to India via KLIA in the Maastrichtian-Paleocene. Whereas, the seasonally dry lineages of Linum migrated from Eurasia into India probably during the Oligocene when the climatic conditions were dry and warm arid. The decline in global temperature towards the end of Eocene escalated the diversification rates within Linaceae that is currently found in the regions of meso- and microthermal climate. The next major shift in diversification rate was detected on the crown node of North American Linaceae during the Middle Miocene Climate Transition.