Cytotaxonomic Studies in Apocynaceae and Delineation of the Different Evolutionary Tendencies Operating within the Family

Abstract

SUMMARY1. A cytotaxonomic study of the family. Apocynaceae representing 24 genera and comprising of 48 species and varieties has been made. Thirty of these plants representing 11 genera have been karyologically investigated for the first time. In the remaining 18 plants where some cytological informations were available, karyotypic analyses of the mitotic chromosomes have been reported for the first time in 5 and meiotic study in 2 of them. Up-to-date informations available in 87 species and varieties from the works of different authors have been tabulated.2. The chromosome numbers in different genera of this family reported earlier and observed during the present investigation has revealed the presence of the following basic numbers: 8, 9, 10, 11, 12, 16 (8), 18 (9) and 23. About 76 per cent of the plants have the basic chromosome numbers of ten and eleven; 11.9 per cent show nine or multiple thereof; 5.9 per cent show twenty-three; 3.0 per cent show eight or multiple thereof and 3.0 per cent show twelve as their basic chromosome numbers.3. The subfamily Plumieroideae comprises genera with a wide variation in chromosome numbers which range from 2n = 16 to 2n = 92. In the other sub family Echitoideae most of the plants have 2 n = 20 and 22 chromosomes, and a couple of them 2 n = 24.4. New polyploid series have been detected in 3 out of the 24 genera studied, where 3 series were reported earlier. The frequency of polyploid species and varieties is about 12.5 per cent in Apocynaceae, estimated from 48 species and varieties of this project and 10.5 per cent when all the previous reports were taken into consideration. All the polyploid species and varieties are confined to the subfamily Plumieroideae.5. The chromosomes are small ranging from 0.7 μ, to 4.8 μ in length and the plants of the subfamily Echitoideae have, in general, smaller chromosomes having none longer than 3.4 μ.6. The chromosomes have mostly median to submedian and in a few cases subterminal primary constrictions. Considerable variation exists in the karyotype within varieties and between species. Chromosomes with secondary constrictions could be detected in most of the plants varying from 2 to 8 in number.7. On the basis of this cytotaxonomic study, it is evident that the plants of the two subfamilies — Plumieroideae and Echitoideae of SCHUMANN are natural groups conspicuously differing in several characters. The following conclusions have been drawn: The genus Allamanda is quite distinct for its fewer and larger chromosomes. Vinca rosea L. on the basis of its morphological and cytological characteristics should be transferred from Vinca Linn, to Lochnera Reich, and called Lochnera rosea (L.) Reich. The subtribe Alstoniinae is a heterogeneous group as it has plants with different habits and chromosome numbers. It is desirable to separate Lochnera, Vinca and Amsonia into a new subtribe. The status of Plumeria in this subtribe needs a thorough reinvestigation. The position of the genus Thevetia with 2 n = 20 chromosomes, syncarpous pistil and funnel-shaped corolla in the subtribe Cerberineae is ambiguous. The tribe Echitideae may be conveniently divided into the two sub-tribes — all the woody climbers in one and the plants with shrubby habits viz. Strophanthus and Nerium in another. It also seems reasonable to transfer the genus Vallaris from the tribe Parsonsieae to the tribe Echitideae and be placed in the new subtribe with the woody climbers.