Acknowledgements
4.3 Reproductive Mechanisms
Polyembryony
Nucellar embryos
Mangoes can be classifi ed into two groups, monoembryonic and polyembry- onic, based on their mode of reproduction from seeds. In general, monoem- bryonic seeds are found in the sub-tropical group (Indian type) and the polyembryonic seeds in the tropical group (South-east Asian). Monoembry- onic mango seeds each contain a single zygotic embryo, and hence only one seedling per seed that is of probable hybrid origin. Polyembryonic mango
Breeding and Genetics 71
seeds can contain one or more embryos, one of which is usually, but not always zygotic. Adventitious embryos develop from the nucellus, a maternal tissue surrounding the embryo sac, and consequently the seedlings of poly- embryonic mangoes are genetically identical to the maternal parent. Adven- titious embryos can also originate by direct budding from the cotyledons and hypocotyls of other nucellar embryos (Juliano, 1934). According to Mahesh- wari and Rangaswamy (1958), the nucellar cells destined to form adventi- tious embryos are recognizable by their dense cytoplasm and starchy contents. They gradually push into the embryo sac cavity where they divide and differentiate into embryos.
Inheritance of polyembryony
Polyembryony is genetically determined. Leroy (1947) considered that adven- tive embryony probably refl ects the effect of one or more recessive genes.
This view was supported by Sturrock (1968), whose study of the progenies of monoembryonic mango hybridized with polyembryonic cultivars indicated that monoembryony was possibly a dominant trait. In contrast, Aron et al.
(1998) and Brettell et al. (2004) observed that polyembryony in mango is con- trolled by a single dominant gene. Schnell et al. (2006) reported that 58 of the Florida cultivars had been classifi ed with 50 being monoembryonic and eight polyembryonic. Information from the Florida cultivars parentage analysis using 25 microsatellite markers supported the fi ndings of Aron et al. (1998) where polyembryony was found to be dominant. ‘Haden’ is a cross of the monoembryonic ‘Mulgoba’ and the polyembryonic ‘Turpentine’. If we assume that a single dominant gene controls this trait, all of the Indian cultivars in Florida must be homozygous recessive and the ‘Turpentine’ parent of ‘Haden’
must have been heterozygous. The evidence suggests that ‘Haden’ inherited the recessive allele from ‘Turpentine’, as all identifi ed progeny of ‘Haden’ are monoembryonic with the exception of ‘Winters’. The most probable pollen parent of ‘Winters’ is ‘Ono’, a polyembryonic cultivar from Hawaii. The fre- quency of this dominant allele is low in the Florida population and absent from the Indian cultivars in Florida. In view of these interesting fi ndings, and since a thorough knowledge of inheritance of polyembryony is essential for speculating the origin of M. indica, more work on these lines is warranted.
Floral biology and pollination
The mango infl orescence is primarily terminal, although axillary and multi- ple panicles may also arise from axillary buds. Both perfect (hermaphrodite) and staminate (male) fl owers occur in the same infl orescence. The total num- ber of fl owers in a panicle may vary from 1000 to 6000, depending on the cultivar (Mukherjee, 1953). Initial fruit set in mango is directly related to the proportion of perfect fl owers, although the fi nal fruit set does not necessarily depend on this ratio (Iyer et al., 1989). It appears that the proportion of per- fect fl owers in a cultivar becomes critical for optimum fruit set only when the proportion drops to 1%.
C.P.A. Iyer and R.J. Schnell 72
Flowers begin to open early in the morning and anthesis has generally been completed by noon. The greatest number of fl owers opens between 9 and 10 a.m. Although the receptivity of the stigma continues for 72 h after anthesis, it is most receptive during the fi rst 6 h; however, there are reports that the stigma can become receptive even before anthesis has occurred (Singh, 1960). The minimum time required for pollen grains to germinate is 1.5 h (Sen et al., 1946; Singh, 1954; Spencer and Kennard, 1955). Singh and Singh (1952) observed 98% pollen viability after 11 months in storage at 7°C and 25% relative humidity (RH), and 65.7% viability after 24 months of stor- age at 0°C and 25% RH.
Mango is cross-pollinated, which is carried out by insects such as the common housefl y, honeybees and thrips, and possibly by other insects al-though to a lesser extent. Pollination by wind and gravity has been sug- gested to occur in mango (Popenoe, 1917; Maheshwari, 1934; Malik, 1951). In nature, > 50% of fl owers do not receive any pollen. Some workers had sug- gested that self-pollination in certain cultivars can also occur quite frequently (Dijkman and Soule, 1951). Studies by Issarakraisila and Considine (1994) have shown that for polyembryonic ‘Kensington’, a night temperature of
< 10°C results in pollen grains with low viability (< 50%). The optimum temperature for normal meiosis is between 15 and 33°C with 70–85%
viability.
Incompatibility
Although the existence of self-sterility in mango was suspected several years ago (Ruehle and Lynch, 1948, cited in Sharma and Singh, 1970; Dijkman and Soule, 1951), the prevalence of self-incompatibility was clearly established in monoembryonic ‘Dashehari’ by Singh et al. (1962). Subsequently, detailed studies indicated that the four popular monoembryonic cultivars of northern India (i.e. ‘Dashehari’, ‘Langra’, ‘Chausa’ and ‘Bombay Green’) were self- incompatible (Mukherjee et al., 1968; Sharma and Singh, 1970). Embryo- logical studies have shown that although fertilization takes place after self-pollination, degeneration of endosperm occurs 15 days after pollination involving self-incompatible parents (Mukherjee et al., 1968). The self- incompatibility system operating in mango appears to be of the sporophytic type. Instances of cross-incompatibility among certain mango cultivars have also been reported (Ram et al., 1976), necessitating the identifi cation of suitable pollinizers for mango.
Using an approach involving isozyme analysis, Dag et al. (2006) have initiated studies in many commercial mango cultivars in Israel and con- cluded that self-pollination is not a yield-limiting factor in monoembryonic
‘Maya’ and the practice of planting ‘Maya’ in solid blocks is sound. They had obtained similar results earlier with monoembryonic ‘Tommy Atkins’ (Dag et al., 1997).
Breeding and Genetics 73
Cytology
Chromosome number
Information on the cytology of mango is quite limited. Only a few Mangifera species (i.e. M. indica, M. caloneura, M. sylvatica, M. foetida, M. caesia, M. odorata and M. zeylancia) have been studied, and were found to have chromosome numbers of 2n = 2x = 40 and n = x = 20 (Mukherjee, 1950, 1957; Roy and Visweswariya, 1951). Chromosome numbers and ploidy status of other species are yet to be studied. The only exception to this chromosome number that has been reported to date (Roy and Visweswariya, 1951) involves ‘Vallikolamban’, which was reported to be tetraploid (2n = 4x = 80), although subsequent stud- ies have indicated that it is only a diploid (Majumder and Sharma, 1990).
Polyploidy
Mango has been referred to as an allopolyploid. Due to the presence of secondary associations at metaphase of meiosis, Mukherjee (1950) suggested that the basic chromosome number of Mangifera is n = 8. In addition, the high number of somatic chromosomes and the correspondingly high number of nucleolar chromosomes led him to conclude that mango is an allopolyploid.
However, the evidence used to arrive at this conclusion is not unequivocal.
In fact, the molecular marker evidence is antithetical to this conclusion.
Results from Duval et al. (2005), Viruel et al. (2005) and Schnell et al. (2005, 2006) using microsatellite markers all indicate that M. indica is diploid.
Although many wild Mangifera species are potentially valuable for crop improvement, they are yet to be exploited. Mukherjee (1963) felt that the different Mangifera species could intercross easily, based on the success ob- tained with interspecifi c crosses between M. zeylanica and M. odorata.