COLUMBUS, J. TRAVIS*, MICHAEL S. KINNEY, MARIA ELENA SIQUEIROS DELGADO, AND ROSA CERROS TLATILPA. Rancho Santa Ana Botanic Garden, Claremont, CA 91711. - Homoplasy, polyphyly, and generic circumscription: the demise of the Boutelouinae (Gramineae: Chloridoideae).
In 1986 Clayton and Renvoize (Kew Bull., Addit. Ser. 13: 1-389)
assembled 16 genera into the Boutelouinae, one of their four subtribes
in tribe Cynodonteae (Gramineae: Chloridoideae). However, evidence
we have gathered from morphology (including lemma micromorphology),
leaf blade transverse anatomy, cytology, and nuclear and chloroplast
DNA sequences indicates that the subtribe, and indeed the tribe, is
polyphyletic. Aegopogon and Schaffnerella appear
closely related to genera in tribe Eragrostideae--Pereilema and
Lycurus, respectively. The affinities of Melanocenchris
and Neobouteloua remain uncertain, as is the case for the clade
comprised of the sister taxa Hilaria and Pleuraphis
(often treated as a single genus, as by Clayton and Renvoize). The
remaining 12 Boutelouinae genera form a monophyletic group, but the
largest genera--Bouteloua s.s. and Chondrosium--are not
monophyletic. Considering our much-improved knowledge of
relationships, the features in common to the 12 genera, and the
practical issue of identification, we advocate that the group be
treated as a single genus (Bouteloua s.l.). The many generic
synonyms are testament to the wide range of variation in inflorescence
form--including sexual dimorphism and branch
persistence/abscission--displayed by Bouteloua. The evidence
shows that some architectures have evolved in parallel, notably the
Chondrosium-like staminate inflorescence of species scattered
in several lineages. The long and erroneous affiliation of
Aegopogon with Bouteloua, especially the
Cathestecum group of species, can be owed to homoplasy.
Remarkable is the convergence in inflorescence form between
Aegopogon and the Cathestecum group: spikelets three
per branch, the lateral two reduced; branches short, pendulous, and
deciduous; lemma 3-awned. Although macromorphology has generally
served us well in the classification of grasses and other plants,
other sources of data are necessary to detect cases of homoplasy and
to ensure that the units of classification best reflect phylogeny.
Key words: Aegopogon, Bouteloua, Boutelouinae, Chloridoideae, Chondrosium, Schaffnerella