Uneven Diploid Chromosome Numbers and Complex Heterozygosity in Homeria (Iridaceae)

Abstract

Complex translocation heterozygotes have been discovered in three species of Homeria, a genus of southern African Iridaceae. These species, H. tenuis (2n = 10, 9), H. fiavescens (ined., 2n = 9), and H. pallida (2n = 8) are the only aneu- ploid species or races in the genus; H. pallida also has forms with 2n = 12. Basic chromosome number in Homeria is x = 6 and most species are diploid, a few poly- ploid. The genus comprises mostly large-flowered outcrossing species, but self-com- patibility and autogamy occur in several, including the three with complex hetero- zygosity. Observations on meiotic behavior in the species mentioned are incomplete but established in H. pallida in which alternate segregation of chromosomes regularly occurs. In all species pollen fertility is low, 55% or less, but abundant seed is pro- duced. Homeria is a southern African genus of some 30 species, all geophytic, corm-bearing perennials (Goldblatt in press). Species occur in a variety of habitats from desert to Mediterranean shrubland (fynbos) to grass- land. Most species are large-flowered, obligate outcrossers but a few, some with small, very short-lasting flowers, are self-compatible and au- togamous. Basic chromosome number in the genus is x = 6 (Goldblatt 1971, 1980) and the species with x = 6 have a karyotype consisting of large acrocentric chromosomes. The majority of species of Homeria are diploid with 2n = 12 (18 species), a few have both diploid and polyploid forms, and four species are apparently entirely polyploid with 2n = 24 or 36. The only known exceptions are H. pallida Baker, which has races with 2n = 12 and 2n = 8; H. tenuis Schltr., in which one population has been found with 2n = 10 and three with 2n = 9; and H. flavescens Goldbl. ined. in which 2n = 9 has been found in two populations. All three species are autogamous. The diploid number 2n = 9 is apparently characteristic of whole popu- lations, as indicated by random sampling of four to eight individuals per population. The reduction in chromosome number (fig. 1) is associated with struc- tural rearrangements of the Robertsonian type (Jones 1977, 1978). Thus the karyotype in H. tenuis with 2n = 10 has 8 acrocentrics and 2 large metacentrics, and H. pallida with 2n = 8 has 4 acrocentrics and 4 large