Female-biased Inheritance in Tetraploid Hemp

Abstract

Hemp(Cannabis sativa) is naturally diploid and dioecious with 1:1 female-to-male progeny. However, female-biased seed is desired for dual-purpose fiber and grain production to maximize yield. Based on investigations from the early 1940s, tetraploidy may be a way to produce female-biased seed. We evaluated sex phenotype ratios and genotype ratios by polymerase chain reaction allelic competitive extension using CSP-2 primers through F1 and F2 generations to ascertain the level of female bias from tetraploids. F1 progeny (n 5 102) from crossing XXXX 3 XXYY had a phenotypic ratio of 7.5:1 female/predominantly female plants-to-male plants. Plants scored as predominantly female were very similar visually to those scored as females and had an equivalent number of flowering nodes, but they produced a small number (< 10) of male flowers that occurred on a small number of lateral nodes (two to five, with less than five flowers per node). F1 plants were allowed to cross-pollinate in the greenhouse, and F2 seed from one female and one predominantly female plant, with confirmed genotypes of XXXX and XXXY, respectively, were evaluated. Inheritance patterns for F2 progeny (n 5 51) from XXXX 3 XXYY indicated preferential chromosome pairing behavior during meiosis for XY gametes, which resulted in 84% XXXY plants. Inheritance patterns for F2 progeny (n 5 51) from XXXY 3 XXYY did not match those expected for tetrasomic segregation and were inter-mediate, with a ratio of 3:1 female/predominantly female plants-to-male plants. Genotype ratios fit a model in which XY ova fertilized by XY or YY sperm were inviable, possibly resulting from arrested endosperm development when the X:Y balance was # 1. A small degree of self-pollinating of predominantly female F1 plants may have affected F2 inheritance patterns. Tetraploidy may be a useful approach for breeding female-biased seed resulting from a strong X-to-autosome balance system in hemp. Because the predominant F1 genotype from the XXXX 3 XXYY cross is XXXY, a female bias of 3:1 and as high as 5:1 could be expected for market-ready F2 seed, which is a significant shift from 1:1 for diploid hemp.