The monophyly of the group comprising the core malvalean families, Bombacaceae, Malvaceae,
Sterculiaceae, and Tiliaceae, was recently confirmed by molecular studies, but the
internal structure of this clade is poorly understood. In this study, we examined
sequences of the chloroplast ndhF gene (aligned length 2226 bp) from 70 exemplars
representing 35 of the 39 putative tribes of core Malvales. The monophyly of one traditional
family, the Malvaceae, was supported in the trees resulting from these data, but the
other three families, as traditionally circumscribed, are nonmonophyletic. In addition,
the following relationships were well supported: (1) a clade, /Malvatheca, consisting
of traditional Malvaceae and Bombacaceae (except some members of tribe Durioneae),
plus Fremontodendron and Chiranthodendron, which are usually treated as Sterculiaceae;
(2) a clade, /Malvadendrina, supported by a unique 21-bp (base pair) deletion and
consisting of /Malvatheca, plus five additional subclades, including representatives
of Sterculiaceae and Tiliaceae, and Durionieae; (3) a clade, /Byttneriina, with genera
traditionally assigned to several tribes of Tiliaceae, plus exemplars of tribes Byttnerieae,
Hermannieae, and Lasiopetaleae of Sterculiaceae. The most striking departures from
traditional classifications are the following: Durio and relatives appear to be more
closely related to Helicteres and Reevesia (Sterculiaceae) than to Bombacaceae; several
genera traditionally considered as Bombacaceae (Camptostemon, Matisia, Phragmotheca,
and Quararibea) or Sterculiaceae (Chiranthodendron and Fremontodendron) appear as
sister lineages to the traditional Malvaceae; the traditional tribe Helictereae (Sterculiaceae)
is polyphyletic; and Sterculiaceae and Tiliaceae, as traditionally circumscribed,
represent polyphyletic groups that cannot sensibly be maintained with their traditional
limits for purposes of classification. We discuss morphological characters and conclude
that there has been extensive homoplasy in characters previously used to delineate
major taxonomic groups in core Malvales. The topologies here also suggest that /Malvatheca
do not have as a synapormophy monothecate anthers, as has been previously supposed
but, instead, may be united by dithecate, transversely septate (polysporangiate) anthers,
as found in basal members of both /Bombacoideae and /Malvoideae. Thus, "monothecate"
anthers may have been derived at least twice, independently, within the /Bombacoideae
(core Bombacaceae) and /Malvoideae (traditional Malvaceae).