Seed grown Cannabis is notoriously variable due its dioecious and obligate out-crossing nature. While breeding efforts have stabilized important agronomic traits, significant genetic heterozygosity remains within cultivars, causing detrimental phenotypic variation. Therefore, uniform, vigorous plants grown from genetically uniform F1-hybrid seed is a major aspiration in Cannabis breeding.
The creation of phenotypically stable, homozygous parental lines is key to this effort. In addition to serving as parents for F1 hybrid seed, genetically fixed homozygous lines facilitate breeding efforts, particularly in backcrossing and mutagenesis programs, and constitute important genetic resources in pre-breeding studies such as quantitative trait loci analysis, functional gene characterization and genome mapping. Inbreeding is an effective method of fixing desirable traits as well as increasing homozygosity of each successive generation. Serial inbreeding via single seed decent (SSD) is currently the only realistic pathway to create true pure-breeding homozygous parental lines in Cannabis, which could be crossed to create the desired uniform F1-hybrid seed.
The consistency of sex expression is an important consideration in this breeding method, both as an economically valuable trait in its own right and because the lines need to remain fertile through the generations. However, there are reports that inbreeding Cannabis induces changes in sex expression and loss of fertility due to inbreeding depression, which would influence the feasibility of reaching homozygosity through serial self-fertilisation. This study aims to create a series of homozygous lines through single seed descent inbreeding to understand if sex expression and fertility are altered through successive inbreeding and to assess the performance of F1-hybrids of the resulting crosses.