Species relationships and genetic analysis of isozymes in aster occidentalis, aster falcatus and the allotetraploid aster ascendens
Date
1996
Authors
Eccleston, Cindy Lyn
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Abstract
Complex species relationships within the genus Aster have proven a challenge to systematists for decades. This study examines these complexities with an investigation into evolutionary relationships among three species of Aster. Aster occidentalis (x = 8) and Aster falcatus (x = 5) are believed to be the progenitor species of the allotetraploid species Aster ascendens (x = 13). Previous morphological and cytological analysis show that A. ascendens resembles A. occidentalis in ray colour and growth habit, and A. falcatus in the shape of the outer phyllaries, and that the karyotype of A. ascendens exhibits a combination of karyotype characteristics from both putative parent species. The purposes of this study were: (1). to determine isozyme inheritance patterns and linkages in nine polymorphic loci of A. occidental is and A. falcatus through analysis of testcrosses in each species using starch gel electrophoresis and enzyme specific staining, and (2) to determine the degree of divergence among these three species using genetic analysis and to test the hypothesis of hybrid origin. Seventeen populations of A. occidentalis (n=8), A. falcatus (n=5), and A. ascendens (n=13) were examined for allozyme variation at 15 polymorphic loci representing 10 enzyme systems.
Inheritance patterns for nine loci ( coding for six of the ten enzyme systems studied) were determined using controlled testcrosses. Chi - square (x2) analysis of A. occidentalis testcrosses revealed linkage between Adh-1 and Pgi-2 and between Mdh-1 and Pgm-2. Three additional loci (Lap-I, Tpi-1 and Tpi-2) were found to assort independently, providing markers for five linkage groups in total. Testcrosses of A. falcatus were not successful Linkage analysis of A. falcatus and A. ascendens would be of great value in the construction of a genetic map for these three species and would be useful in further clarification of the relationships among them.
Allozyme studies showed that A. ascendens and A. occidentalis had a higher genetic identity than A. ascendens and A. falcatus. Genetic identities for A. ascendens and A. occidentalis (0.834), and for A. ascendens and A. falcatus 0.648) were intermediate to identities among populations within species and the identity between A. occidentalis and A. falcatus (0.539). A. ascendens shares 9 alleles with A. occidentalis, 5 alleles with A. falcatus and 34 alleles with both parents out of 66 alleles examined. Unique alleles were found in all three species. Results of this genetic analysis support findings based on previous morphological and cytological studies and suggest that hybridization either came about long enough ago to allow new alleles to appear in A. ascendens, or occurred more than once in different geographic areas with different alleles present in the parent species.