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?