Confirmation of cytotype stability in autotetraploid black wattle (Acacia mearnsii) trees using flow cytometry and size differences of the reproductive gametes

Abstract

Acacia mearnsii (black wattle) is grown commercially in South Africa for its timber and bark. However, the invasive nature of the species has resulted in it being considered an alien invader and for this reason research has been aimed at producing a sterile triploid variety that would be highly desirable for the South African commercial forestry industry. Tetraploids were successfully induced by soaking germinating diploid seeds in colchicine. Seed from these tetraploids was used to establish a field trial, where crossing diploid with tetraploid parent plants only produced diploid and tetraploid progeny and failed to produce any triploid progeny. Control-crossed seed set between diploids is generally low in A. mearnsii and, together with the possibility of an unstable tetraploid population, this could be reducing the chances of producing triploid seed. Thus identification and confirmation of stability within the existing advanced-generation tetraploid population (aged 10–11 years) was critical to ensure the production of sterile triploids. Flow cytometry was used to determine the stability of the ploidy of leaf vegetative tissues, whereas polyad and ovule size measurements were used to determine the stability of ploidy of the reproductive tissues. Results from the study revealed that the tetraploidy of within the leaf vegetative tissue was stable. For both the ovule and polyad size measurements, a size range was determined for diploids and tetraploids and, within the population under investigation, no overlap was apparent. This allowed for the conclusion that the advanced-generation tetraploid population was stable and that the absence of triploid progeny must be because of post-zygotic reproductive barriers within the ovary.