Chromosome numbers, pollen fertility, and pollen size in Haworthia species and hybrids

Sporophytic chromosome numbers were determined for 216 species, varieties, or hybrids of the genus Haworthia. There were 152 diploids (2n = 14), 10 triploids, 38 tetraploids, 7 pentaploids, 7 hexaploids, and 2 aneuploids. Among the species studied, most of the diploids were highly fertile. The tetraploids were somewhat less so, the triploids were generally less than 50% fertile, and the pentaploids and hexaploids were generally intermediate between the diploids and the triploids. The few diploid interspecific hybrids were less fertile than the diploid species. Each additional set of chromosomes increased the pollen size, and this increment was found to be significant up to the tetraploid level. Statistical analyses of comparisons between pollen size means of the hexaploids and pentaploids, and of the pentaploids and tetraploids were not significant. Pollen sterility is interesting in connection with the problem of speciation in the Aloineae, which is discussed.