Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] has been improved for yield and other traits as part of the process of plant breeding. Knowledge of the rate (percentage) of self- or cross-pollination is useful for watermelon breeders interested in planning isolation distances, estimating components of genetic variance, or selecting among progenies produced through open-pollination. In cross-pollinated crops, it is often assumed that individuals produced from a single parent are half-sib families and those genetic variances should be calculated on that assumption. However, variances may be improperly estimated if there is natural self-pollination (inbreeding). In addition, knowledge of the rate of natural self- or cross-pollination in crops is useful in designing experiments for genetic studies, crop improvement, and for maintaining elite inbred lines (Chowdhury and Slinkard, 1997).
Studies of several crops, including barley (Hordeum vulgare L.), lima bean (Phaseolus lunatus L.), wild oat (Avena fatua L.), and rose clover (Trifolium hirsutum All.), all assumed to be predominantly self-pollinating species, have shown that even low outcrossing rates of 1% to 10% had a significant effect on the genetic structure of the populations (Harding and Tucker, 1964; Jain, 1976). Crop improvement methods for self-pollinated crops are different from those of cross-pollinated crops (Fehr, 1993). Common methods for crop improvement used in watermelon are pedigree breeding and recurrent selection (Wehner, 2008). Breeders often work with large F2 populations to recover improved trait combinations for individual plant selections using pedigree breeding. In cross-pollinated crops, controlled self-pollination is made on individual plants by covering flowers before they open, requiring resources for each population and family to be advanced. If the rate of self-pollination can be increased in the field, watermelon breeders can harvest open-pollinated seeds from individual plant selections in early generations. On the other hand, if the rate of natural outcrossing can be increased, watermelon populations can be improved by recurrent selection by using natural intercrossing of selected families in isolation blocks (Kumar and Wehner, 2011). Intercrossing can play an important role in genetic gain (Wehner and Cramer, 1996).
Crop species are classified as autogamous, allogamous, or mixed mating types. Watermelon is predominantly an allogamous species with monoecious or andromonoecious flowering habit (Ferreira et al., 2002). The a locus determines sex expression in watermelon, producing monoecious (AA) or andromonoecious (aa) sex expression (Guner and Wehner, 2004; Martin et al., 2009; Rhodes and Dane, 1999; Rhodes and Zhang, 1995). Monoecious sex expression promotes allogamy, whereas andromonoecious sex expression can promote autogamy (Martin et al., 2009). Cucurbits often grow as single plants or small populations in the wild. That leads to inbreeding and selection against inbreeding depression as well as the elimination of deleterious recessive alleles (Allard, 1999). In previous studies, there is no significant inbreeding depression measured in watermelon (Wehner, 2008). Further support for that was provided by Ferreira et al. (2000, 2002) who reported an inbreeding coefficient as high as 0.41 and outcrossing rate of 65% in andromonoecious families of watermelon and 77% averaged over monoecious and andromonoecious families. Thus, although watermelon is assumed to be a cross-pollinated crop, there is significant self-pollination that breeders should be aware of. Natural self-pollination can be of some use in a low-resource breeding program.
Pollination in watermelon is mediated by honeybees (Apis mellifera L.) and bumblebees (Bombus impatiens Cresson) that visit flowers to collect pollen and nectar (Delaplane and Mayer, 2000; Free, 1993; McGregor, 1976). The movement of honeybees and bumblebees among flowers in a field is directional, within rows rather than across rows (Cresswell et al., 1995; Handel, 1982; Walters and Schultheis, 2009; Zimmerman, 1979). The directional movement of pollinators within rows may reduce the revisits of flowers and maximize foraging efficiency (Collevatti et al., 2000).
Watermelon breeders often use 3 × 3-m spacing when working with single-plant hills in their breeding program (Neppl and Wehner, 2001). The 3 × 3-m plant spacing provides good separation of vines for pollination and selection. Walters and Schultheis (2009) reported that watermelon plants were mostly self-pollinated when spaced more than 10 m apart. However, 10-m hill spacing may not be economical in breeding programs that handle thousands of plants per year. It might be possible to manipulate the mating behavior (pollination) of watermelon plants by optimizing plant spacing. Furthermore, close plant spacing may be used to enhance cross-pollination, thus facilitating intercrossing among families in a recurrent selection program. Recurrent selection is predominantly used to improve quantitative traits (Hallauer and Miranda, 1988). On the other hand, increasing the plant spacing might increase the amount of self-pollination for individual plant selection where required in methods such as pedigree breeding.
The environment affects pollen flow in cucurbit crops which, in turn, affects the rate of self- or cross-pollination (Gingras et al., 1999; Stanghellini and Schultheis, 2005). Variation in wind velocity, humidity, light intensity, temperature, and other environmental factors over years and locations may influence pollinator behavior and sex expression in watermelon and thus affect the rate of self-pollination (Kalbarczyk, 2009; Robinson and Decker-Walters, 1997). Self-pollination has been reported to vary from 23% to 77% over locations in cucumber (Wehner and Jenkins, 1985). Jenkins (1942) reported 30% to 35% natural self-pollination in cucumber. Hence, it is useful to study the rate of self- or cross-pollination over multiple years and locations.
In tomato and watermelon, cultivars differ in their ability to produce pollen (Lesley, 1924; Stanghellini and Schultheis, 2005). Pollination may be affected by the amount of pollen produced by the flowers of a particular cultivar. The availability of more pollen to the pollinators may increase the rate of outcrossing. The mating behavior in watermelon depends on environment, cultivar, and flight pattern of pollinators. However, we were interested in those aspects of mating behavior (pollination) that are under the control of the plant breeder. The objective of this study was to determine the rate of self- or cross-pollination in watermelon as affected by spacing, year, location, and cultivar.
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