Major structural changes are remarkably frequent in the chromosomes of Aloe (Brandham, 1976; Brandham and Johnson, 1977). These are sometimes paracentric or pericentric inversions, but more frequently take the form of inter-chromosomal movements of segments of material of different lengths. They are either transposed from one chromosome to another or mutually exchanged between chromosomes, and are often large enough to alter the gross morphology of the chromosomes involved. They can also appear as Robertsonian interchanges, in which two of the acrocentric or submetacentric chromosomes break at or near the centromere and re-join to produce a short metacentric from the two short arms and a very long metacentric from the two long arms (Brandham, 1976).
Although these structural changes have been shown to be very common in wild populations of some species, as found in mapped and analysed populations of A. pubescens Reynolds and A. rabaiensis Rendle, they are never found in the homozygous condition (Brandham and Johnson, 1977). They are always in the heterozygous condition, with a single long fusion chromosome and a single short one, the homologues retaining their normal appearance. There seems to be selection against the formation of homozygotes in the wild, even though they have been produced in cultivation (Brandham, 1983). A homozygote for a Robertsonian change involving two pairs of long chromosomes of a diploid aloe will comprise three pairs of short acrocentrics as normal, two pairs of long acrocentrics/submetacentrics, one pair of short metacentrics derived from the fused short arms and one pair of very long metacentrics derived from the fused long arms. Such an individual would thus have a karyotype that is new for Aloe, even though the basic number of chromosomes remains unaltered. It would be bivalent-forming at meiosis, fully fertile and genetically isolated from non-interchange plants. If it were to appear in the wild, it would form the origin of a new group with the potential to develop into new species with a karyotype not typical of the genus. Being always heterozygous in the wild, these major structural changes in Aloe chromosomes do not stabilise widely; in fact, they do not contribute to the genetic differentiation of species and should therefore be regarded as 'evolutionary noise' (Brandham, 1983; Brandham and Doherty, 1998).
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