Search

In the oasis of Tozeur (Tunisia) a peculiar date palm ploidy chimera was discovered, showing a diploid (2n = 2x) and a tetraploid (2n =4x) sector and a uniform tetraploid offshoot. This chimera allowed to describe the effects of chromosome doubling on some selected features. Because both ploidies occur within one plant, there was no environmental bias. Tetraploid leaves are longer, bear larger leaflets and spines and have a thicker rachis than normal diploid leaves. During three seasons, tetraploid inflorescences appeared ten days later than their diploid counterparts. The tetraploid sector formed bunches with stalks bearing larger fruits, with a higher percentage of inedible parthenocarpic fruits, than bunches that arose from the diploid sector. Tetraploid fruits are significantly longer, wider and heavier than diploid fruits, with a larger pulp diameter, irrespective of whether they are seeded or parthenocarpic. Diploid parthenocarpic fruits are longer than seeded fruits, while tetraploid parthenocarpic and seeded fruits both show the same size. The occurrence of tetraploidy opens new perspectives in date palm breeding. Pollination of tetraploid pistils with pollen from diploid males gave rise to a relatively low number of triploid plantlets, suggesting a triploid block.

An allopolyploid complex with high genomic integrity has been studied. Dogroses transmit only seven chromosomes (from seven bivalents) through the pollen, whereas 21, 28 or 35 chromosomes (from seven bivalents and 14, 21 or 28 univalents) come from the egg cells. Seedlings derived from two interspecific crosses were analysed with flow cytometry and molecular markers to determine ploidy level, mode of reproduction and genomic constitution. Evidence was obtained for the formation of unreduced male and female gametes, which can take part in fertilization (producing seedlings with higher ploidy than the parental plants) or in apomictic reproduction. Random amplified polymorphic DNA (RAPD) and microsatellite analyses indicated that three seedlings (5%) were derived through apomixis, whereas the other 49 were hybrids. Bivalent formation appears to involve chromosomes that consistently share the same microsatellite alleles. Allele-sharing between the maternally transmitted and highly conserved univalent-forming chromosomes reflected the taxonomic distance between different genotypes. The frequently recombining bivalent-forming chromosomes were taxonomically less informative.