Gene number in species of Astereae that have different chromosome numbers

Abstract

Differencesin the gametic chromosome numbers (n = 4, 5, 9) ofspecies.in the Astereae tribe ofthe Compositae have been variously .interpreted. One hypothesis proposes that n = 9 was the original base number ofthe group and that the lower numbers resulted from aneuploid reduction. The alternative hypothesis asserts that the ancestral base number was n = 4 or n = 5 and that species in which n = 9 are allotetraploids derived by hybridization between taxa with the lower numbers. Electrophoretic analysis of 17 enzyme systems' in.five species of Machaeranthera, in which n = 4, 5, and 9, and two species of Aster in which n = 5 and 9,-demonstrates that all of these species have the same number of gene loci specifying the tested enzymes. The absence of isozyme multiplicity in the species in which n = 9 suggests that they did not arise by polyploidy.Ploidy level in plants has usually been determined primarily on the basis ofchromosome number. Although satisfactory-in most cases, sharp differences characterize the interpretation ofchromosome numbers in the Astereae tribe of the Compositae. The most common chromosome number in the tribe is n = 9, but the numbers vary between n = 2 and n = 9, with many species having n = 4 or n = 5. One hypothesis proposes that n = 9 was the original base number of the group and that lower numbers resulted from aneuploid reduction (1). Evidence claimed to support this view is the association of n = 9 with the primitive woody habit, the high symmetry of the n = 9 karyotypes, and the widespread occurrence of this chromosome number in divergent lineages within the tribe. The alternative hypothesis calls attention to the rarity of species with the intermediate chromosome numbers n = 6 and n = 7 and asserts that the ancestral base number was n = 4.or n = 5, so that species in which n = 9 are allotetraploids derived by hybridization 'between taxa with the lower numbers (2, 3).The essential attribute ofpolyploidy, however, is not relative chromosome number but genome multiplication and its attendant increases in number ofgene loci. Consequently, I have used gel electrophoresis to determine whether species in which n = 9 have more gene loci coding particular isozymes than those in which n = 4 or 5. The approach builds on two recent findings: (i) allopolyploid species display isozyme multiplicity relative to diploids because they inherit from their diploid parents homoeologous loci that are frequently fixed for different alleles (4-6) and (ii) decrease in chromosome number from the ancestral diploid level as a result ofaneuploidy does not change the number of structural genes specifying isozymes (7). Thus, .if species in which n = 9 have many more isozyme loci than those in which n = 4 or n = 5, they are likely to be polyploid. Polyploidy is rejected if they have the same number ofloci. (Jackson 7640, Beatty NV). The seeds were generously provided by R. C. Jackson. Several-ofthe collections had been previously grown for electrophoretic~studies (8). MATERIALS AND METHODSThe seeds were germinat...

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