Cytogenetics

Chromosome number

Like many other Bromeliaceae, all the botanical varieties of A. comosus have a diploid number of 50 minute, almost spherical, chromosomes, with a length of 0.5-1.7 p,m. Among three pineapple accessions, Sharma and Ghosh (1971) identified four chromosome types and observed a slight variation in their distribution and in chromatin content. They also observed secondary constrictions in two to five pairs of chromosomes. Arumuganathan and Earle (1991) reported values of 1.09 and 0.92 pg (corresponding to c. 526 and 444 Mbp) for the DNA content of 2C nuclei of A. comosus var. comosus and A. comosus var. bracteatus, respectively. Triploids (e.g. cultivars 'Cabezona', 'Caicara', 'Monte Oscuro'), tetraploids and even heteroploids (60 chromosomes) may also occur (Collins and Kerns, 1931; Canpinpin and Rotor, 1937; Collins, 1960; Antoni, 1983; Lin et al, 1987; Dujardin, 1991). Collins and Kerns (1931) reported six cases of triploidy out of 8000 plants resulting from a cross between 'Smooth Cayenne' and a clone of A. comosus var. ananassoides. Collins (1933b) demonstrated that these triploids had arisen from unreduced female egg cells fertilized by normal haploid pollen. Tetraploids could then be obtained from unreduced male and female gametes, although their occurrence is rarer than triploids. These tetraploids produce about 90% viable pollen, of a larger size than pollen cells of diploid plants. By crossing them with diploids, hybrid tetraploid seedlings were obtained, which grew faster and larger than diploids. A. macrodontes is a natural tetraploid with 100 chromosomes (Collins, 1960).

Ploidy level and morphology

Collins (1933b, 1960) and Kerns and Collins (1947) compared triploid and diploid hybrids and diploid and autotetraploid plants of 'Smooth Cayenne'. The polyploids were taller, with wider leaves, placed at a greater internodal length, so the total leaf area does not differ much. The cross-section of the tetraploid leaves is more closed. Their larger fibres increase leaf rigidity. The size of cells, trichomes and stomata also increases with the ploidy level, while the stomata density is reduced. Water content is higher in polyploids. Triploid hybrids and tetraploid 'Smooth Cayenne' have longer vegetative growth periods, mature more slowly and have a lower fruit sugar content. Tetraploid fruits are smaller, with fewer but larger fruitlets.

Micro- and megagametogenesis and fertility

Meiosis is primarily regular in diploid and tetraploid A. comosus, as well as in A. macrodontes, with only rare irregularities, such as lagging chromosomes and some abnormal tetrads (Collins and Kerns, 1931; Canpinpin and Rotor, 1937; Collins, 1960). However, some irregular gametes are produced. The proportion of giant pollen cells may reach 6.5% and that of giant ovules 1%. Bhowmik (1977) observed large variations in size and fertility of pollen grains in 'Kew' ('Smooth Cayenne') and 'Queen' (from 35 to 81 ^m and from 36 to 68 ^m, respectively) and correlated them with the frequencies of univalents and secondary associations of up to three or seven bivalents in these cultivars. Pollen size and fertility also vary between cultivars and clones (Collins, 1960; Ramirez, 1966; Wee and Rao, 1979; Nayar et al., 1981; Antoni, 1983; Coppens d'Eeckenbrugge et al., 1993). Triploid genotypes are sterile because of irregular meiosis. Tetraploids produce 90% of good pollen grains, which are uniform but larger than those of diploids (Collins, 1960).

The number of ovules varies widely in A. comosus. While A. comosus var. bracteatus may have 40-70 ovules per flower, this number generally ranges from 14 to 30 in the wild botanical varieties and in A. comosus var. erec-tifolius and from 16 to 71 in A. comosus var. comosus (Coppens d'Eeckenbrugge et al.,

1993). There is also variation in all the species in the relative frequencies of anat-ropous and orthotropous ovules. The embryo sac develops according to the Polygonum type. Shrinking embryo sacs have been commonly observed at the eight-nucleate stage in 'Masmerah' (Rao and Wee, 1979) and the six main cultivars of A. comosus var. comosus and in A. comosus var. ananassoides, A. comosus var. erectifolius and A. comosus var. bracteatus (F. Van Miegroet, personal communication). The proportion of ovules with normal embryo sacs is correlated with the proportion of stained pollen and with fertility (F. Van Miegroet and Coppens d'Eeckenbrugge, unpublished results).

Fertility rate (i.e. the percentage of ovules producing a seed) is generally lower in A. comosus var. comosus than in the other botanical varieties. In the latter, fertility ranged from 6% (0.85 seeds per flower) in A. comosus var. ananassoides to 35% (18.4 seeds per flower) in A. comosus var. bracteatus. Within A. comosus var. comosus, the cultivars with 'piping' leaves often exhibit a higher fertility (4-11%; two to five seeds per flower) than the common cultivars 'Smooth Cayenne', 'Española Roja', 'Singapore Spanish', 'Pérola' and 'Queen' (less than 5%; zero to two seeds per flower). However, the highest fertility rate (29%; 9.45 seeds per flower), which is comparable to that of wild types, was observed in a spiny landrace. These fertility rates were estimated under an open-pollination situation and higher values may be expected under assisted pollination. Fertility is correlated with ovule fertility, pollen stainability and the amount of pollen produced per flower. It is not correlated with ovule number, probably due to crowding or competition effects between fertilized ovules. Indeed, many genotypes of A. como-sus var. comosus and A. comosus var. bractea-tus can develop only a small proportion of their numerous ovules into seeds (Coppens d'Eeckenbrugge et al., 1993). Fertility may be affected by abnormal anthesis. Thus, in certain clones of 'Española Roja' and 'Singapore Spanish', a significant proportion of flowers do not open, hampering pollination.

Self-incompatibility

In A. comosus, the self-incompatibility is brought about by the inhibition of pollentube growth in the upper third of the style (Kerns, 1932; Majumder et al, 1964). It is gametophytically controlled by a single locus with multiple alleles (Brewbaker and Gorrez, 1967) and is generally stronger in A. comosus var. comosus than in the other botanical varieties. Later work, however, suggested that more than one gene might be involved in the control of self-incompatibility (D.D.F. Williams, Colorado, 2001, personal communication). However, many cultivars exhibit pseudo-self-compatibility, expressed in the variable production of self-seeds, although the resulting self-fertility is always lower than cross-fertility. For instance, pseudo-self-incompatibility is common in 'Singapore Spanish', 'Perolera', 'Manzana', 'Primavera', 'Rondon', 'Cambray', 'Amarelo de Uaupés', 'Roxo de Tefé', 'Morada' and 'Alto Turi' and in some clones of 'Queen' (DeWald et al., 1992; Coppens d'Eeckenbrugge et al., 1993, 1997; Muller, 1994). In a sample of 84 clones from 51 cultivars and primitive landraces studied by Muller (1994), 55 appeared strongly self-incompatible and 21 pseudoself-compatible and eight exhibited a self-fertility comparable to their fertility, which could be due to self-compatibility or a very weak self-incompatibility. Indeed, a dominant mutation suppresses the incompatibility reaction in the pollen of 'Smooth Cayenne' and results in the production of selfed seeds (Collins, 1960). The self-incompatibility of A. comosus var. comosus does not appear to be affected by tetraploidy (Kerns and Collins, 1947).

Pseudo-self-compatibility is more frequent in the wild pineapples. In Muller's study, five out of 11 A. comosus var. ananas-soides clones and four out of seven A. comosus var. parguazensis clones exhibited pseudo-self-compatibility. One A. comosus var. par-guazensis clone appeared self-compatible. In A. comosus var. bracteatus, three clones that were phenotypically identical appeared to be self-compatible. A variegated clone of the same species was self-incompatible, while another was pseudo-self-compatible. A. macrodontes is highly self-fertile and produces uniform progenies, suggesting that this species is autogamous.

Hybridization between botanical varieties and between species

There is little difficulty in crossing cultivars of A. comosus var. comosus with clones from the other botanical varieties. Instead, these crosses often show high fertility, as a result of higher gamete viability of the latter. When A. comosus is crossed with A. macrodontes, a few fertile seeds are produced, which usually yield vigorous tetraploids and sometimes self-sterile triploids. The tetraploid hybrids are highly fertile and self-fertile (Collins, 1960).

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