Cytological and Biochemical Aspects of Elm Improvement

Abstract

As far as we know, the American elm (Ulmus americana L.) is the only natural tetraploid species in the genus, with 2n=4x=56 chromosomes. It was thought that doubling the chromosome number of Siberian elm (U. pumila L.) would allow interspecific hybridization between these two species. However, no hybrids with American elm were developed by using the colchiploid Siberian elms as either the male or female parent. Two naturally occurring triploid elms, one a putative hybrid between U. americana and U. pumila and the other a derivative of U. pumila and U. rubra Muhl. parentage, have been reported in the literature. Both of these triploids produced viable pollen grains and were used as male parents in crossing on U. parvifolia Jacq. Progenies of these tri-specific hybrids, as well as open-pollinated progeny of the U. pumila x U. rubra triploid, included a wide range of aneuploids, most of which did not have acceptable tree form or were susceptible to Dutch elm disease. Recently, another triploid elm (of unknown ancestry) and two diploid American (?) elms have been discovered. Detailed cytological analyses of these trees and their progenies would be desirable. Furthermore, there is a need for biochemical data that will help to make more accurate assessments of the botanical origins of these atypical plants.