Unfruitfulness

Gynoecious and andromonoecious varieties are preferred because they consist of female and hermaphrodite plants that produce marketable fruit for fresh consumption. These populations are derived from crosses between hermaphrodite plants in a ratio 2:1 in segregation of female and hermaphrodite plants. This is somewhat advantageous for producers, since only the hermaphrodite plants remain in the plantations after the definitive sexing procedure, once consumers prefer fruits of hermaphrodite plants (Khan et  al. 2002). Recent molecular studies have shown that sex determination in this species is controlled by a recently evolved homomorphic pair of sex chromosomes (type X/Y), differentiated by a small male-specific region on the Y chromosome (MSY). In this sense, the female plants are homogametic with the XX chromosomes, whereas male and hermaphrodite plants are heterogametic, with XY and XYh chromosome combinations, respectively (Liu et al. 2004). It is likely that two genes are involved in papaya sex determination, the first gene is a suppressor of the stamen in female flowers (feminising gene) and the other gene is a suppressor of carpel in male flowers (masculinising gene) (Ming et al. 2007). More recent research has detected seven genes in the sex-controlling region, which were however not able to differentiate the three sex types, because there was neither differential expression nor dosage effect, suggesting that these genes are not involved in sex determination (Yu et al. 2008a, b). Despite certain progress in research on the molecular mechanisms of papaya sex determination, there is little understanding of the expression of the sexual forms and variations, which are directly related to the production efficiency of marketable fruit. This issue is considered complex and intriguing in view of the lethal factor associated with the dominant alleles responsible for hermaphroditism and masculinity (Ming et  al. 2007), aside from the influence of genetic and environmental factors on both (Damasceno Junior et  al. 2008). The combination of these factors is possibly responsible for the high degree of instability of hermaphrodite plants in terms of sex expression; a variation of flowers to carpelloid and pentandric

forms or sex reversal is possible. These variations of the hermaphrodite flower, classified as floral abnormalities, reduce crop yields and increase seasonality in fruit yield, leading to supply oscillations and consequently to price variations of papaya on the market. Carpelloid fruits occur due to the transformation of stamens into carpel-like structures, producing fruit with varying degrees of malformation, while pentandrya is the transformation of hermaphrodite into a typically female flower, with a reduced number of stamens and an ovary with five deep grooves. These variations occur mainly at mild or low temperatures and high moisture and soil nitrogen levels (Awada 1958; Arkle Junior and Nakasone 1984) and are undesirable in plantations, since their fruit has no market value, decreasing yields. Sex reversal, however, is a result of abortion or ovarian atrophy, producing no fruit. This phenomenon occurs related to high temperatures, water stress and low soil nitrogen (Awada 1958; Arkle Junior and Nakasone 1984).