Cytology, Crossability, and Pollen Fertility of Sri Lankan Exacum (Gentianaceae) and Their Hybrids

Chromosome numbers and sexual reproduction were evaluated in the Exacum trinervium complex (subsp. trinervium, subsp. ritigalensis, subsp. macranthum, and subsp. pallidum) and the closely related species E. pedunculatum and E. affine. Primary hybridizations between taxa with common chromosome numbers produced many capsules and viable seedlings, while hybridizations between taxa with different chromosome numbers also produced capsules and viable seedlings but at reduced rates. Chromosome numbers were determined to be as follows: E. trinervium subsp. trinervium, 2n ¼ 60; subsp. ritigalensis, 2n ¼ 60; subsp. macranthum, 2n ¼ 54; subsp. pallidum, 2n ¼ 52; and E. pedunculatum, 2n ¼ 56. Exacum affine is confirmed as 2n ¼ 36. No capsules or seedlings were produced when E. affine was hybridized with any Sri Lankan taxon. Pollen from the Sri Lankan taxa ranged in viability from 34% (subsp. macranthum) to 43% (subsp. trinervium), while that from primary hybrids depended on the parental taxon combination. Our results support the conspecificity of subsp. trinervium and subsp. ritigalensis, with the latter remaining a subspecies within E. trinervium, and lend some support to the rank of species for subsp. macranthum and subsp. pallidum, depending on how strictly the biological species concept is interpreted. Although we reaffirm the species status of E. pedunculatum, a limited number of fertile hybrids were produced between it and several other taxa, suggesting a closer taxonomic relationship than previously proposed.