At present there is karyological information on ca 10% of the species and ca 30% of the genera of the Apocynaceae. Basic numbers of x = 6, 8, 9, 10, 11, 12, 16, 18, 20, 21 and 23 have been assessed. Of these x = 11 is primitive, occurring in ca 60% of the genera.
Those of x = 6, 8, 9 and 10 have evolved by reduction, and x = 12 by increase from x = 11. In the subtribe Secondatiinae however, x = 12 is most likely the result of doubling x = 6. The numbers x = 16, 18 and 20 are likewise doubles of x = 8, 9 and 10 respectively. Those of x = 21, 23, and in one case, x = 20 are probably aneuploid products of doubles of x = 11.
The two larger subfamilies, Plumerioideae and Apocynoideae have the basic numbers x = 8, 9, 10 and 11 in common and are not separable on the basis of chromosomal evidence. The third small subfamily Cerberoideae is more homogeneous according to basic number, i.e. x = 10 and 20. Most genera are characterized by a constant basic number, but some have two basic numbers; these clearly are cases of infrageneric aneuploidy. Based on records in the literature two closely related generaApocynum andTrachomitum appear to be characterized by a basic number of x = 8 as well as x = 11. This conflicting situation should be clarified by further karyological research. From the level of subtribe onwards some taxa have one basic number, but others are characterized by two or more numbers. The occurrence of similar basic numbers in different phylads of the family is considered to be the result of similar chromosomal evolution mechanisms.
Approximately 22% of the investigated species are polyploid. Intrageneric polyploidy occurs with a frequency of about 12.5% and infraspecific polyploidy with less than 4%.
The karyotypes observed are symmetrical: the chromosomes within a karyotype are similar in length with primary constrictions usually in a median position. In the Tabernaemontaneae however, it was observed that the karyotypes comprise one pair of distinctly heterobrachial chromosomes in addition to the metacentric ones. This tribe is also characterized by chromosomes which are relatively long.
Most genera of the African continent, which are well known regarding their chromosome number, are characterized by x = 11. Exceptions areStrophantus (x = 9) with a mainly tropical African distribution. Two other genera with derived numbers, i.e.Gonioma with x = 10 andPachypodium with x = 9, occur in southern Africa and Madagascar. The genera with a non-African distribution are less known for their chromosome number. However, the available evidence suggests that evolution of derived numbers has occurred more frequently outside Africa than on this continent.
KeywordsChromosome Number Evolution Mechanism Basic Number African Continent Chromosomal Evolution
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