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