There are two basic theories about how sex is determined in Cannabis. The epigamic (non-genetic) theory holds that sex is determined by physiological stimuli at some stage after fertilization. This is based on the study of sex _reversal in changing environmental conditions. The alternative theory is that sex in Cannabis can be explained simply in terms of sex inheritance of the XY type. Since the X and Y chromosomes do not differ sufficiently in size to distinguish them easily by direct observation, there is further temptation to consider sex determination epigamic. However, genetic analysis of polyploids indicates the XY mode of sex determination does take place in Cannabis to some degree.

 

The research of Warmke and Davidson (1943) and Zhatov (1979) involved polyploid strains of Cannabis. Warmke and Davidson (1943) discovered that the sex ratio for diploid (2n) crosses very nearly approaches 1-to-1 but the sex ratio for tetraploid (4n) crosses is approximately 7.5 pistillate and pistillate-hermaphrodite plants to each staminate plant. A new sex class, XXXY, is formed, and a shift in sex ratio of the F1 tetraploid generation results. Two possible explanations exist for the high number of pistillate plants in tetraploid crosses:

 

1 - The new XXXY class is pistillate or pistillate-hermaphrodite and not easily distinguishable from the XXXX type.

 

2 - The pistillate plant, rather than the staminate plant, is heterogametic XY. In this case XXXY and XXYY would be grouped together and appear as pistillates or pistillate-hermaphrodites, the XXXX type being staminate. However, McPhee (1925) found good genetic evidence that

the female is not heterogametic in Cannabis. Warmke and Davidson confirmed these results by selfing a partially hermaphroditic pistillate plant; the offspring were predominantly pistillate. It seems, therefore, that the second theory to explain the excess of females is unlikely: if the first theory is correct, there are two classes of F1 tetraploid pistillate offspring, XXXX and XXXY.

 

Tetraploid pistillate plants (XXXX) when crossed to diploid staminate (XY) plants yield nearly 100% pistillate and pistillate-hermaphrodite individuals. In 31 crosses of F1 tetraploid pistillate plants (XXXX and XXXY) with diploid staminate plants, 2 resulted in 98% pistillate and pistillate-hermaphrodite offspring and 29 resulted in 75% pistillate and pistillate-hermaphrodite offspring. It seems from these results that XX, XXX, and XXXX individuals are pistillate, XXXY and XXY individuals are pistillate-hermaphrodite; and XY, XXY and XXYY individuals are staminate.

 

Zhatov (1979) also found that in populations of tetraploid Cannabis many transitional sexual types emerge both in growth habit and ratio of staminate and pistillate flowers. Since the genetic complement is doubled, the subsequent generations of tetraploids exhibit more intermediate sexual types than diploids. He determined that XXXX individuals are pistillate, XXXY individuals are pistillate hermaphrodite, XXYY individuals are monoecious, XYYY individuals are staminatehermaphrodites, and YYYY individuals are staminate. However, at the diploid level it is more difficult to explain the occurrence of monoecious strains and hermaphrodites. Monoecious strains beget monoecious offspring in a great majority of instances. A strictly XY determination of sex does not explain monoecious strains. Monoeciousness could be controlled by another gene or set of genes, separate from the basic XY determination of sex. Hermaphrodism in dioecious strains is most likely controlled by a number of genes for separate aspects of floral induction.

 

The epigamic approach rejects any chance that sex is determined by genetics, while the genetic approach is incompatible with any environmental control of sex and the occurrence of monoecious strains. It seems that we must incorporate both theories to come to a workable understanding of sexual expression in Cannabis.

 

The most logical accommodation is to consider the initial sexual characteristics of Cannabis, such as sexual dimorphism of pre-floral plants and primordial differentiation, to be determined by an XY type of genetic inheritance. Although the initial sexual form is determined, the final production of floral organs is influenced by other genes and by environmental conditions which may override the expression of the inherited sexual type. The effect of environment could change the chemical make-up of the plant; for example, carbohydrateto-nitrogen ratios and fluctuations in metabolic levels in the cytoplasm might alter or mask the chemical interpretations of inherited sexual traits by messengers within the cytoplasm. Many tropical drug strains from Africa and Southeast Asia turn hermaphrodite in temperate climates. This is probably a reaction to an introduced environment.