Topodeme differentiation in Ontario taxa of Crataegus (Rosaceae: Maloideae): leaf morphometric evidence

Variation in the two-dimensional shape of short-shoot subterminal leaves was examined quantitatively in samples of nine topodemes of Crataegus section Crus-galli Loud. (representing C. crus-galli L. s.str., C. fontanesiana (Spach) Steud., C. ?disperma Ashe, and C. ?grandis Ashe) plus three of section Punctatae Loud. (C. punctata Jacq.). Shape was described by 23 linear measurements comprising a network of box trusses. These measurements were summarized by means of sheared principal components analysis, so as to obtain two axes maximizing different size-independent contrasts in shape among the 12 samples. Topodemes of section Crus-galli were more differentiated from one another and, on the whole, less variable than those of C. punctata. These results resemble those obtained earlier with these taxa, with flower and fruit data. A probable explanation for these results is the contrasting reproductive behavior found in C. section Crus-galli (agamospermy, self-compatibility) and C. punctata (apomixis absent, self-incompatible). Such an explanation also illuminates the possible origin of much of the taxonomic complexity of the genus Crataegus in North America.