Isozyme Evidence for the Naturalness of Mespilus L. (Rosaceae, Subfam. Maloideae)

Abstract

Isozyme phenotypes for 13 enzyme systems were obtained for both species of Mespilus, as well as for a number of species in related genera of subfam. Maloideae. Comparison of these phenotypes revealed that the two Mespilus species grouped together with isozyme analysis, an interesting observation in view of their wide geographic separation. Similarly, each of the other genera examined displayed its own characteristic isozyme phenotype. These results support the inclusion of M. canescens within Mespilus. The recent discovery and description (Phipps 1990) of a second species of Mespilus L., M. ca- nescens Phipps, endemic to the U.S.A., raises intriguing questions about the systematics and biogeography of the genus. Mespilus has di- verged little from Crataegus L., a much larger genus in the tribe Crataegeae. Mespilus canescens is not only quite distinct from the previously known M. germanica L. but is separated from it by a considerable geographic distance. Mespilus germanica is native to western Asia and south- east Europe whereas M. canescens is only known from one locality in Arkansas, U.S.A. Further data bearing on the systematics of Mespilus would, therefore, be very valuable. This paper presents the results of isozyme analysis con- ducted in the Dept. of Horticultural Science, Cornell University, New York while one of us (JBP) was on sabbatical leave. The pertinent taxonomic situation to date is as follows: Mespilus is not strongly demarcated from Crataegus morphologically. Indeed, until about 1830, species were almost randomly de- scribed in either genus. However, once the con- cept of Mespilus, narrowed to that of its type species, M. germanica, crystallized there has been essential unanimity in the criteria for separat- ing the two genera. Both Mespilus and Crataegus have always been placed in the tribe Crataegeae since its erection by Koehne (1890) and are gen- erally considered closely related. Mespilus with its two species may be distinguished polythet-