Introduction
The Pleistocene’s climate and environmental oscillations caused fragmentation and expansion of lowland tropical rain forests, mountain forests, and savannas of Upper Guinea and Lower Guinea, Africa (Duminil et al. 2015). These climate oscillations shaped Afro-alpine species distribution, such as Lobelia giberroa, which occurs on mountains with altitudes between 3500 and 6000 m in East Africa (Kebede et al. 2007). Forest fragmentation during the Pleistocene was a driver that gave origin to allopatric speciation of plants and animals. Likewise, forest refugia and rivers contributed to current diversity patterns in West Central Africa (Nicolas et al. 2012). Moreover, during the Pliocene-Pleistocene epochs, there was substantial volcanic activity in West Central African and East Africa, such as the Eastern Arc Mountains of Tanzania and Kenya (Measey & Tolley 2011).
The Cameroon Volcanic Line (CVL) is an 1800 km SW-NE topographical feature that extends from the Gulf of Guinea to onshore central Cameroon (Adams et al. 2015). This volcanic chain was an ancient forest refuge in West Central Africa (Demenou et al. 2020). The CVL comprises plateaus and 11 dormant volcanoes, and Mount Cameroon, a currently active volcano (Chauvel et al. 2005). Only three volcanic peaks in the CVL have elevations above 3000m (Pico Basilé, Mt. Cameroon and Mt. Oku). The geological structure of the CVL is a combination of tectonic and volcanic origins with unequal ages ranging from the middle to late Tertiary (Jesus et al. 2005). The oldest mountains are in the north, with a trend of decreasing age of volcanic activity in the southern area (Missoup et al. 2016). The marked geographical separation and isolation of the mountains are analogous to islands in the sky or sky islands. Sky islands are considered natural laboratories for studying evolutionary patterns and processes that lead to the accumulation of diversity (Cox et al. 2014). Moreover, the sky islands and sky island archipelagos of the Gulf of Guinea and West Central Africa possess an extraordinary diversity of angiosperms (Figueiredo 1994), small mammals (Missoup et al. 2016), and amphibians (Zimkus & Gvoždík 2013). This African region is part of the Guinea biodiversity hotspot (Myers et al. 2000) and is critical for the conservation of endemic species of plants and animals that inhabit the sky islands (Tropek & Konvicka 2009).
Giant lobelias may have experienced rapid diversification in East Africa mountains and subsequently dispersed to West Africa (Knox & Li 2017).Lobelia columnaris Hook f. (Campanulaceae: Lobelioideae) is listed as a vulnerable species in the IUCN Red List of Threatened Species 2015 (Cheek & Thulin 2015). L . columnaris andL . barnsii Exell (Mabberley 1974a) are the two giant lobelia species known from the tropical West Central Africa and Gulf of Guinea. L . columnaris is endemic to Bioko’s mountains and highlands (Equatorial Guinea), Nigeria, and Cameroon. This giant lobelia grows in discontinuous populations between 1000m to 3000m in different ecological habitats including submontane grasslands, forest clearings transformed into grasslands overgrown with Pteridium aquilinum(L.) Kuhn, and along streams and subalpine meadows.
Little is known about the origin and colonization histories of angiosperms in Bioko and Cameroon. However, a few phylogeographic studies have documented the lineages of plants and animals currently present in Bioko and Cameroon. For example, the Afromontane genusLynchis had several dispersals from Ethiopia and the Western Rift Mountains and recently dispersed to West Africa (Popp et al. 2007). The hypothesis of recurrent connections over time between the West and East African mountains provides a framework to discuss the biogeographical origin among close relatives in both bioregions, like the endangered and endemic Mount Oku rat, Lamottemys okuensis in the CVL (Missoup et al. 2016). However, some taxa have a geographically widespread distribution, from the eastern mainland to the outlying islands of western Africa, like the endangered Prunus africana (Dawson & Powell 1999).
The present study’s objective is to infer the phylogenetic relationships of populations of L. columnaris using chloroplast genomes and estimate the divergence time to reconstruct its historical colonization on the sky islands of Bioko and Cameroon. Specifically, we aim to answer the following questions: (1) What is the phylogenetic relationship among Bioko Island and Cameroon populations? (2) Are the older populations found on the older sky islands? (3) Does the colonization history reflect the age of the sky islands?