A Karyological Survey of Asclepiadoideae, Periplocoideae, and Secamonoideae, and Evolutionary Considerations within Apocynaceae s.l.

Abstract

The chromosome numbers of 672 species out of 740 taxa of the Apocynaceae in the subfamilies Asclepiadoideae, Periplocoideae, and Secamonoideae were either determined or extracted from the literature. The chromosome numbers of 299 taxa are published here for the first time, and a further 139 new counts confirm or complete previously published chromosome numbers. The basic chromosome number x = 11 is found to be predominant, occurring in ca. 96% of the taxa investigated. Deviations from x = 11 are absent in Periplocoideae and Secamonoideae, but some variation is present in Asclepiadoideae. A reduction of the karyotype to x = 10 is known only in the advanced tribe Asclepiadeae, where it is important in the Old World subtribe Astephaninae (Microloma), as well as in the New World subtribes Metastelminae (Funastrum, Orthosia, Philibertia, and Grisebachiella) and Oxypetalinae (Araujia and Tweedia). An increase (x = 12, 13, 14) was found only rarely and sporadically in the subfamily Asclepiadoideae. About 6% of the species (7% of the taxa) investigated are found to be polyploid. The majority of such taxa are tetraploid with 2n = 44; only a few are hexaploid or higher. The data on chromosome number and size presented in this paper are interpreted in relation to the systematic position and geographical distribution of the taxa investigated. On the average karyotype size diminishes from rather large chromosomes in the Periplocoideae to the smallest karyotype length in the presumed most advanced tribe of the Asclepidoideae, the Asclepiadeae. Reduction in the basic chromosome number is regarded as only a rather recent step in genome evolution within Asclepiadeae, which has been taken place in parallel in the Old and New Worlds. Finally, comparision with data available for the Apocynaceae s. str. (Apocynoideae and Rauvolfioideae) is presented.