Gomphocalyx and Phylohydrax (Rubiaceae): sister taxa excluded from Spermacoceae s.s., featuring a remarkable case of convergent evolution

Abstract

The genera Gomphocalyx and Phylohydrax (Rubiaceae) have been invariably placed in the tribe Spermacoceae s.s. based on the uni-ovulate ovary locules and pluri-aperturate pollen grains. Sequence data from the rps16 intron and the rbcL gene here presented exclude Gomphocalyx and Phylohydrax from Spermacoceae s.s. The two genera are closely related and their nearest relatives are found among members of the former tribe Hedyotideae. This position may be surprising at first, because the growth form of Phylohydrax is similar to some observed in Spermacoceae s.s. A detailed survey of morphological and anatomical characters shows, however, that the character states of the two genera are largely consistent with the here-proposed position in Hedyotideae. The creeping growth form, the uni-ovulate ovaries and the pluri-colporate pollen grains of Gomphocalyx and Phylohydrax are the result of convergence between this pair of genera and Spermacoceae s.s. The taxonomic position of Lathraeocarpa, sometimes considered closely related to Gomphocalyx, is also discussed.

Systematics, taxonomy and floristics of Brazilian Rubiaceae: an overview about the current status and future challenges

“…Salas & E.L. Cabral from Staelia, based on palynological and morphological comparisons, but no dedicated phylogenetic studies were performed in order to test the monophyly of these taxa. In conclusion, following the generic concepts adopted by Bremer & Manen (2000), , , Delprete (2007Delprete ( , 2010c, Dessein (2003), Dessein et al (2005), Groeninckx et al (2009a,b), Lens et al (2009), and Karehed & Bremer (2007) (Kirkbride 1997), Bradea (Brade 1949), Denscantia (Cabral & Bacigalupo 2001a,b), Diacrodon (Sprague 1928), Diodella Bacigalupo & Cabral 2006;Bacigalupo & Cabral in Borhidi 2006), Diodia (Bacigalupo & Cabral 1999), Galianthe (Cabral 1991(Cabral , 2002(Cabral , 2005Cabral & Bacigalupo 1997;Delprete 2010a), Leptoscela (Hooker 1873), Manettia (Macias 1998), Mitracarpus (Souza 2008;Souza et al 2010), Psyllocarpus (Salas & Cabral 2010a,b)), Standleya (Brade 1932(Brade , 1949, and Schwendenera (Bacigalupo & Cabral 2007b). …”

“…2004;Dessein 2003;Lantz 2003;Motley et al 2005;Persson 2000a,b;Piesschaert et al 2000;Razafimandimbison & Bremer 2001, 2002Rova et al 2002;Stoffelen et al 1996), with the intent of clarifying the subfamilial, tribal and generic delimitation in the Rubiaceae. Following these studies, a family classification for a general overview on the systematic significance of pollen in the Rubiaceae was published by Dessein et al (2005), where significant correlations with tribal delimitations were detected.…”

“…The relationships between the Spermacoceae sensu stricto and the Hedyotideae have been studied by many rubiologists. Recent molecular phylogenies (Bremer & Manen 2000;Dessein 2003;Dessein et al 2005;Karehed & Bremer 2007) demonstrated that this complex should be treated as a single, monophyletic tribe (including Hedyotideae and Manettieae). Also, the generic delimitations within the Spermacoceae sensu lato are the most complex and debated within the Rubiaceae, and are still far from being resolved.…”

Phylogeny of the Herbaceous Tribe Spermacoceae (Rubiaceae) Based on Plastid DNA Data1

“…However, molecular data show Spermacoceae s. str. to be deeply nested within the Hedyotideae, making the latter tribe paraphyletic (Bremer, 1996;Andersson & Rova, 1999;Bremer & Manen, 2000;Dessein et al, 2005a). Therefore, Bremer (1996) and later Bremer and Manen (2000) proposed a wider definition for Spermacoceae, in which the former tribes Spermacoceae s.…”

“…as a more prudent taxonomic approach to handle the information from molecular-based analyses. The latter view was followed by Dessein (2003), who preferred to recognize an emended tribe Knoxieae (including Knoxieae s. str., Triainolepideae, Otiophora, the Pentas group of Hedyotideae fide Dessein et al [2000], and Carphalea Juss.)…”