EVANS, RODGER C.* AND CHRISTOPER S. CAMPBELL. Department of Biological Sciences, University of Maine, Orono, ME 04469-5722. - The polyploid origin of a large clade: Nuclear Granule-Bound Starch Synthase (GBSSI or waxy) gene sequences support a spiraeoid ancestry of the Maloideae (Rosaceae).
Base chromosome number (X) is 7, 8, or 9 in most members of Rosaceae
except the Maloideae (X =17). The base number of 17 plus molecular
and morphological data support the inclusion in Maloideae of
Kageneckia , Lindleya , and Vauquelinia (X
=15), which were traditionally assigned to the polyphyletic
"Spiraeoideae." The base number of 17 has long been
interpreted as evidence of either allopolyploidization involving
ancestral Amygdaloideae (X=8) and "Spiraeoideae" (X=9) or
polyploidization within ancestral "Spiraeoideae". To
investigate the origin of Maloideae, we cloned and sequenced 941 bp
from nine exons in the 5’ portion of GBSSI. We sampled 13 genera of
Maloideae (including Kageneckia , Lindleya , and
Vauquelinia ) plus genera of Rosaceae that previous studies have
shown to be closely related to the Maloideae. Our analysis supports a
close relationship between our Maloideae sample and X = 9
Porteranthus ("Spiraeoideae") of the southeastern
United States. Maloideae have four GBSSI paralogues, two (GBSSI-1 and
GBSSI-2) from a duplication prior to the origin of Rosaceae and two
from a duplication within each Maloideae clade (GBSSI-1A and B;
GBSSI-2A and B). Multiple clones of Porteranthus nest within
either Maloideae GBSSI-1 or 2 and are weakly linked to GBSSI-1A or
GBSSI-2A. Monophyly of Maloideae- Porteranthus clades is
supported by bootstrap values approaching 100%, loss of the sixth
intron in all GBSSI-1 sequences, alignability of introns between
genera, and numerous non-molecular characters. Our results are
consistent with a polyploid origin involving only members of a lineage
that contained the ancestors of Porteranthus . Under this
hypothesis, the subfamily originated in North America, and the high
Maloideae chromosome number arose via aneuploidy from X = 18.
Key words: GBSSI, low-copy number nuclear gene, Maloideae, phylogeny, polyploid origin, Rosaceae, waxy gene