Are Melilotus albus and M. officinalis conspecific?

The principal economic species of the genus Melilotus are white sweet-clover ( Melilotus albus ) and the extremely similar yellow sweet-clover ( M. officinalis ). Although they are widely recognized as distinct species, some influential references in North America reduce the former to a subspecific rank or even merely a conspecific synonym of the latter. Given their importance and the large numbers of germplasm collections, the doubt needs to be resolved. This review of relevant published evidence finds that in addition to the difference in floral colour, the traditional segregation of the two as distinct species is best supported by very strong reproductive barriers as well as divergent DNA sequences in three barcoding genes. Additional but weaker confirmation of separateness is provided by studies reporting differences in external morphology, biochemistry, seed protein profiles, karyotype and DNA microsatellites.