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