Chromosomes in Sphaeralcea and Related Genera

Abstract

The chromosome number has been determined for 15 species and 15 subspecies, representing 10 of the 12 sections of the subgenus Eusphaeralcea. The basic chromosome number is 5. The occurrence of 5, 10 and 15 pairs is frequent, but only one form with 25 pairs has been found. No form having 20 pairs has been discovered as yet.Among approximately 275 plants examined, 22 were judged, from their morphological characters, to have originated as natural hybrids. Four of these apparent hybrids exhibited chromosome irregularities typical of F1 hybrids, 5 showed irregularities such as are characteristic of segregating hybrid generations, and 13 showed normal meiotic behavior.Six species and one subspecies of the related genus Malvastrum (subgenus Malacothamnus) were found to have 17 pairs of ehromosomes.Of forms usually referred to Sphaeralcea, S. rivularis (subgenus Iliamna) has 33 pairs of chromosomes and S. umbellata (subgenus Meliphlea), and S. abutiloides (subgenus Phymosia) have 17 pairs.The basic chromosome number of 5 in Eusphaeralcea is the lowest yet reported in the Malvaceae, and indicates that there is insufficient evidence that the ancestral basic number for the family is 7. The polyploid series 5, 10, 15 and 25 pairs is of exceptional interest, since it occurs in the wild and is the largest series known in the family.In Eusphaeralcea no relation is apparent between the chromosome number and the taxonomy of the group. The frequent occurrence of natural hybrids and their meiotic behavior, indicate a rather closely related group of species.The chromosome number of the subgenus Malacothamnus of Malvastrum clearly separates this group from Sphaeralcea. The chromosome numbers of S. rivularis, S. umbellata and S. abutiloides indicate that the genera Iliamna, Meliphlea and Phymosia may be well founded.