Interchange and inversion polymorphism among populations of Haworthia reinwardtii var. chalumnensis

Abstract

In a survey of 145 triploid and tetraploid plants of Haworthia reinwardtii var. chalumnensis from eleven localities in South Africa hybridity for twelve asymmetrical reciprocal chromosome interchanges and two pericentric inversions has been found, occurring spontaneously with a frequency unprecedented in the Aloineae. These events occur singly or as two or three per individual, and no plant has an unaltered Aloineae karyotype. No position effect of interchange on morphology could be detected, but evidence was obtained for an adaptive effect, heterozygotes being selected preferentially in sexually reproducing populations. The geographical distribution of the interchanges correlates with the probable point of origin and the direction of spread of the taxon. It was found that the distribution of chromosome breaks among the four long chromosomes of the basikaryotype is at random, but that the distribution of breaks within a chromosome is not. Centric breaks greatly exceed randomness, and it is suggested that mechanical tension rather than background radiation is the major factor causing spontaneous chromosome breakage, resulting in the weak point of the chromosome, the centromere, breaking in preference to other regions of the chromosome.