Pleistocene origin and colonization history of Lobelia columnaris Hook.
f. (Campanulaceae: Lobelioideae) across sky islands of West Central
We aimed to infer the phylogenetic relationships of populations of
Lobelia 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? We assembled novel plastomes from 20 individuals of L.
columnaris from five mountain systems. The plastome data was explored
with phylogenetic analyses using Maximum likelihood and Bayesian
Inference. The complete plastome size varied from 164,609 bp to 165,368
bp. The populations of L. columnaris have a monophyletic origin,
subdivided into three plastome-geographic clades. The plastid
phylogenomic results and age of the sky islands indicate that L.
columnaris colonized first along the Cameroon Volcanic Line’s young sky
islands. The earliest divergent event (1.54 Ma) split the population in
South Bioko from those on the mainland and North Bioko. The population
of South Bioko was likely isolated during cold and dry conditions in
forest refugia. Presumably, the colonization history occurred during the
middle-late Pleistocene from South Bioko’s young sky island to North
Bioko and the northern old sky islands in Cameroon. Furthermore, the
central depression with lowland forest between North and South Bioko is
a current geographic barrier that keeps separate the populations of
Bioko from each other and the mainland populations. The Pleistocene
climatic oscillations led to the divergence of the Cameroon and Bioko
populations into three clades. L. columnaris colonized the older sky
island in mainland Cameroon after establishing South Bioko’s younger sky
islands. The biogeography history was an inverse progression concerning
the age of the Afromontane sky islands.