Female-biased Inheritance in Tetraploid Hemp

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 F 1 and F 2 generations to ascertain the level of female bias from tetraploids. F 1 progeny (n = 102) from crossing XXXX × 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). F 1 plants were allowed to cross-pollinate in the greenhouse, and F 2 seed from one female and one predominantly female plant, with confirmed genotypes of XXXX and XXXY, respectively, were evaluated. Inheritance patterns for F 2 progeny (n = 51) from XXXX × XXYY indicated preferential chromosome pairing behavior during meiosis for XY gametes, which resulted in 84% XXXY plants. Inheritance patterns for F 2 progeny (n = 51) from XXXY × XXYY did not match those expected for tetrasomic segregation and were intermediate, 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 F 1 plants may have affected F 2 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 F 1 genotype from the XXXX × XXYY cross is XXXY, a female bias of 3:1 and as high as 5:1 could be expected for market-ready F 2 seed, which is a significant shift from 1:1 for diploid hemp.