Morpho-cytological studies and DNA barcoding in genus Senna (Fabaceae, Caesalpinoideae)

Abstract

Senna Mill. is a complex plant group known to have underlying biological ambiguity, due to the high similarity that exists among its morphological attributes consequently making identification of closely related species very difficult. Considering the medicinal and nutritional values of some members of this genus, it is therefore important to employ a reliable and robust technique to delimit the members of this plant group. The present study employed morphological characters, chromosome counts and matK and rbcL primer markers to delimit and elucidate the phylogenetic relationship that exists among the seven Senna species from Ile-Ife, Nigeria. The cluster analysis of the studied morphological traits grouped S. podocarpa, S. alata and S. siamea on one hand. At the same time, the result showed that S. occidentalis, S. hirsuta, S. obstusifolia and S. sophera were closely related on the other hand. A chromosome count of x=18 was recorded in S. podocarpa and S. obstusifolia, x=20 in S. occidentalis x=22 in S. alata and S. hirsuta, and x=24 in S. siamea and S. sophera. There was a 100% yield in PCR amplification of the Senna species studied. The DNA sequences ranged from 546 to 843 base pairs and the phylogenetic tree revealed that S. podocarpa was genetically distant from the other species. Moreover, the phylogenetic tree resolution showed similarities among the Senna species studied, the Nigerian Senna and other Senna species referenced sequences obtained from NCBI. Despite similarities among these seven Senna species, they could be distinguished by the morphological attributes employed for their characterization. There is an addition of x=10 to the already known basic chromosome numbers in Senna. The chromosome numbers were greatly varied from previous counts and therefore have limitations as taxonomic tools.