Further disintegration and redefinition of Clerodendrum (Lamiaceae): Implications for the understanding of the evolution of an intriguing breeding strategy

The genus Clerodendrum s.l. is polyphyletic. Although recent studies have resulted in C. subg. Cyclonema and C. sect. Konocalyx being removed to the resurrected genus Rotheca, and the unispecific genus Huxleya being sunk into Clerodendrum it has been nuclear whether Clerodendrum as currently circumscribed is monophyletic, particularly in relation to the American genera Aegiphila, Amasonia, and Tetraclea. This phylogenetic study employs four relatively fast-evolving chloroplast DNA regions, trnT-L, trnL-F trnD-T, and trnS-fM, to clarify the generic boundaries of Clerodendrum and its relationships to allied genera. The resutls corroborate previous studies that there are three well-supported clades in the currently recognized Clerodendrum: an Asian clade, an African clade, and Pantropical Coastal clade. The Asian clade and African clade are sister groups and together form a monophyletic group. However, the Pantrpical Coastal clade is more closely related to the three American genera than it is to the other two Clerodendrum clade. In addition, a Caribbean species. C spinosum, is found to be more closely related to the American genera than it is to any of the three major Clerodendrum groups. These resuls indicate that Clerodendrum as currently circumscribed is not monophyletic. We propose to separate the Pantrpical Coastal clade and C. spinosum by reviving the genera Volkameria (including Haxleya) and Ovieda, respectively for these, and to restrict Clerodendrum to the Asian and African clade. Brief descriptions of the genera to be recognized are provided. All neotrpical 'Clerodendrum' taxa are referred to other genera, necessistating six new combinations, which are also provided, where required, for two other well-studed Old World Volkameria species; al names ever used in Ovieda are given their modern placing (two placed newly in synonymy). The study also sheld light on the evolution of an intriguising breeding strategy that avoids self-pollination or/and sexual interference. This strategy involves presentation of pollen and stigma in the centre of the flower in a sequential fashon by moving the filaments and sytle. It appears to have evolved in the commno ancestor of Clerodendrum, Volkameria, Ovieda, Amasonia, Tetraclea, Aegiphila and Kalaharia, and still occurs in all the thse taxa except Aegiphila, where it has been succeeded by a heterostylous system.