Cytologically, Aloe and its close relatives are remarkable for the uniformity of the basic number and the gross morphology of their chromosomes. Every species that has been investigated cytologically (the great majority of the described taxa) has the basic number x = 7 and a karyotype that is always bimodal (Brandham, 1971). The haploid karyotype comprises three short acrocentric chromosomes and four long ones, of which three are acrocentric and one, the L1, submetacentric with a more substantial short arm. All of the chromosomes are large compared with those of most angiosperms, with the short ones averaging 1.5—3 pm in length, and the long ones almost exactly 4.6 times larger in a sample of Aloe species (Brandham and Doherty, 1998).
Brandham and Doherty (1998) showed that there is considerable interspecific variation in overall chromosome length in Aloe, as reflected in the nuclear DNA C-value, which ranges in diploid species from 4C = 41.78 picograms (pg) in A. tenuior Haw. to 95.4pg in A. peckii Bally et Verdoorn. Although the number of species examined was not large, there was a strong indication that the primitive species, e.g. A. tenuior, had the lowest nuclear DNA amount (and thus the smallest chromosome set), and that the amount of DNA and overall chromosome size increased with evolutionary advancement. It was noted by Brandham and Doherty (1998) that despite the addition of so much nuclear DNA to the chromosomes of advanced species the ratio of the sizes of their long and short chromosomes was the same as that of the primitive species.
Since the work of Resende (1937), the nucleolar organising or secondary constrictions of Aloe chromosomes have been known to vary in position from one chromosome to another in different species. They usually occur distally on the long arms of one, sometimes two pairs of the long chromosomes, but not always on the same member(s) of the set of four. Less commonly, they are found distally on the short arms of the short chromosomes (Brandham, 1971). These correspond to the 18S-5.8S-26S rDNA sites that have been demonstrated in the same positions by Adams etal. (2000a) using fluorescence in situ hybridization stain technology.
Adams etal. (2000a) also demonstrated numerous other smaller 18S-5.8S-26S rDNA sites that could not be resolved as non-staining gaps with the Feulgen staining method used in earlier studies. These sites were found in several different places in the karyotype, revealing a degree of inter-specific variation in the detailed structure of the chromosomes that had not been suspected earlier.
Adams etal. (2000a) further examined the distribution of 5S rDNA sites and found that, in contrast to the above, all studied species had one in each haploid set placed interstitially on the long arm of the L2. The chromosomes of Aloe are thus very conserved with respect to this feature.
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