SYTSMA, KENNETH J.* and AMY LITT. Department of Botany, University of Wisconsin, Madison, WI 53706; MCDB, Yale University, PO BOX 208104, Yale University, New Haven CT 06520. - Tropical disjunctions in and among the Myrtaceae clade (Myrtaceae, Heteropyxidaceae, Psiloxylaceae, Vochysiaceae): Gondwanan vicariance or dispersal?
The monophyletic clade comprising Myrtaceae, Heteropyxidaceae,
Psiloxylaceae, and Vochysiaceae represents a remarkable example of
tropical / southern temperate disjunctions. Myrtaceae have centers of
diversity in Australasia and South America, but little representation
in Africa; the three species of Heteropyxidaceae are African; the
single species of Psiloxylaceae is confined to the Mascarenes; and
Vochysiaceae are largely tropical American but with two genera and
three species restricted to west tropical Africa. A molecular
phylogenetic analysis (rbcL, ndhF, matK) of the
order Myrtales and with emphasis on this Myrtaceae clade and its
sister group (Melastomataceae, Memecylaceae, Penaeaceae,
Rhynchocalycaceae, Oliniaceae, Alzateaceae, Crypteroniaceae) was
conducted under maximum parsimony and maximum likelihood. Vochysiaceae
are sister to Myrtaceae, Heteropyxidaceae, and Psiloxylaceae, with the
latter two strongly monophyletic. The neotropical Myrtaceae (largely
comprising the subf. Myrtoideae) are well imbedded within a
paraphyletic, capsular-fruited, largely Old World subf.
Leptospermoideae. Sequence evolution in the Myrtaceae clade is
significantly slower than in Melastomataceae and somewhat slower than
in the Crypteroniaceae clade — illustrating generation time effects.
Various molecular dating approaches were used to infer ages of
specific nodes within the Myrtaceae clade of four families using
calibrated fossil dates from Myrtaceae, Melastomataceae, Onagraceae,
and Combretaceae. The results argue for a Gondwanan origin of the
common ancestor of Myrtaceae, Heteropyxidaceae, Psiloxylaceae, and
Vochysiaceae. Within Myrtaceae, the derivation of the several
neotropical and African lineages may well be due to more recent
dispersal. However, the timing of these events is somewhat obscured by
the weak support for the backbone in the cladogram of Myrtaceae. With
the amphi-Atlantic distributed Vochysiaceae now being placed as sister
to the other three families, a vicariance explanation for its
distribution can not be ruled out with the taxon sampling to date.
Key words: dispersal, Heteropyxidaceae, Myrtaceae, Psiloxylaceae, vicariance, Vochysiaceae