Abstract:
Cycads have an interesting evolutionary history since they represent the oldest lineage plants that
originated ~ 300 million years ago (Ma) in the mid-Permian but re-diversified recently around 12
Ma with Cycas and Encephalartos being the most rapidly diversified lineages within the cycads
group. Several studies have explored the taxonomic relationships, diversification history within
the two genera but there’s a limited understanding of evolutionary history, biogeography of
cycads and what drives cycads to extinction risk. The aim of this study was to provide a better
explanation on what predispose cycads to high risk of extinction and also elucidate the
biogeography and evolutionary diversification history of the two cycads genera (African genus
Encephalartos and Asian-Pacific genus Cycas). The three main objectives addressed: firstly, the
ecological factors predisposing cycads to high risk of extinction was explored. Secondly,
reconstruct the most comprehensive phylogeny of the two most diversified cycad genera. Lastly,
investigate the evolutionary and historical biogeography of Encephalartos and Cycas. The risk of
extinction using phylogenetic comparative method and fitting cumulative link mixed model on
biological, ecological and evolutionary data of cycads on the most threatened lineage in the plant
kingdom were investigated. Then, assembled the most complete phylogeny and reconstructed the
historical biogeography of Encephalartos and Cycas using S-DIVA and Binary Bayesian model
(BBM) respectively. The nine group of threats to cycads such as habitat loss, overcollection,
fire, deforestation, medicinal usages, grazing, invasive alien plants, reproduction failure and
the last one linked to climate change impacts flood/drought were identified. But, habitat loss,
overcollection, medicinal uses and reproduction failure were clustered on the cycad tree of life
suggesting that, closely related species might be exposed to similar threats implying that
ecological factors that drives cycads to high risk of extinction were anthropogenically mediated
and resulted in vulnerable (VU) category. The phylogenies of the two genera were found to be
well supported. Encephalartos phylogeny, revealed two major clades following species
geographic origins, one southern African clade and one east-central-west African clade. The
biogeographic analysis suggests that the genus may have diverged around 9 Ma from southern
Africa. Then colonized east-central-west African region through vicariance, suggesting that the
eastern rift system in eastern Africa and the west Africa Dahomey Gap acted as a geographical
barrier limiting species dispersal. Furthermore, most species accumulated in the last 2.6 Ma and
there were no significant shifts in any of the evolutionary events, suggesting that a constant-rate
diversification model is best suited for Encephalartos. The phylogenetic analysis of the Cycas
genus pointed to Indochina as the origin of the genus, which may have dispersed firstly across
the Pacific Islands during the late Miocene aided by multiple excursions of sea levels and the
development of a key innovation (a spongy endocarp) particularly in the seeds of subsection
Rumphiae. The colonization of South China, which was initially thought to be the origin of the
Abstract
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genus, may have occurred more recently aided by both dispersal and vicariance events. Also, no
significant shifts in the evolutionary events that shaped the diversity of Cycas was observed. The
findings of this study provide the evidence of historical biogeography and the evolutionary
events that shaped the current diversity of the genera Encephalartos and Cycas. Overall, this
study is the first to elucidates the pattern of extinction risk in cycads and also to identify that
most threats that drives extinction risk of cycads were anthropogenically mediated.
