American Journal of Botany 46(2): 93-102 (Feb., 1959)
Karyotypes of Iris pumila and related species
L. F. Randolph and Jyotirmay Mitra

ABSTRACT.— The karyotypes of 30- and 32-chromosome geographical variants of the amphidiploid I. pumila from Russia and the Balkans were compared with the karyotype of the typical 32-chromosome Austrian forms of this species and with those of the diploid I. attica and I. pseudo-pumila, previously reported to be the basic species from which I. pumila originated. Plants from 3 collections of a Crimean form of I. pumila with 32 chromosomes had a pair of long chromosomes with submedian centromeres morphologically similar to chromosome 1 of the typical form of I. pumila. In addition, there was another heteromorphic pair of submedian chromosomes with one of the members having a shorter short arm. The manner in which this altered chromosome could have arisen as a result of a heterobrachial inversion is described. Five different collections of I. pumila with 30 chromosomes from Russia differ in several respects from the typical 32-chromosome I. pumila. They have an unusually long pair of chromosomes with a submedian centromere and a secondary constriction in the long arm. This chromosome is the original chromosome 2 which had been altered by the addition of a segment equivalent to the most of the long arm of one of the shorter chromosomes with subterminal centromere. The manner in which this could have occurred as the result of unequal reciprocal translocation is described. Loss of the remaining diminutive portion of the short chromosome with subterminal constriction assumed to have been involved in the unequal interchange of segments producing the modified, longer chromosome 2 would account for the reduction in chromosome number from 32 to 30 in the Russian form of I. pumila. Four pairs of chromosomes with satellites have been found in the 30-chromosome plants whereas 6 pairs of satellited chromosomes are present in the 32-chromosome I. pumila. The spontaneous occurrence of chromosomal alterations of the type here described are considered to be significant factors in the process of chromosomal repatterning resulting in the appearance of new geographical races, and eventually of species of iris, with altered chromosome numbers and modified karyotypes. More specifically it is concluded that amphidiploidy accompanied by chromosomal repatterning resulting from segmental interchange, heterobrachial inversion and related types of chromosomal alterations has played an important role in the evolution of I. pumila and karyological forms of this species occupying different geographical areas.

DISCUSSION.— The evolution of I. pumila as an amphidiploid. Karyotype analyses revealed an amphidiploid origin of I. pumila (2n=32) which combines the diploid set of I. attica (2n=16) and I. pseudopumila (2n=16). The 2 pairs of long and conspicuous metacentric marker chromosomes 1 and 2 characteristic of I. attica and I. pseudo-pumila, respectively, were found to be present in duplicate in I. pumila (Mitra, 1956). Simonet (1934, p. 331) considered I. pumila to be an autotetraploid form of I. attica, as indicated by his statement that "I. attica (n=8) et pumila (n=16) sont respectivement diploides et tétraploides, ce dernier possédant, en double, les éléments characteristiques identifiés chez l'I. attica." It is apparent that Simonet failed to distinguish the difference in position of the centromeres in the 2 pairs of long metacentric chromosomes in I. pumila, due perhaps to limitations of the technique he employed. Since I. attica and I. pumila have various taxonomic characteristics in common it was not illogical to assume that the latter was an autotetraploid of the former.

At the present time, I. pumila has a more northern distribution than either I. attica or I. pseudo-pumila. No 2 of these species are known to be sympatric. However, civilization has encroached very extensively on the habitats of these species in recent centuries and grazing by farm animals is known to have caused the extinction of iris in many areas, leaving them only in restricted, isolated regions. Obviously, the present range of these 3 species is much more restricted than it was before the encroachment of civilization began to limit their distribution