Cytogenetical Studies on Ecological Races of <i>Mecardonia dianthera</i> (SW) Pennel

Abstract

Mecardonia dianthera, a Scrophulariaceous weed of Tropical America comprises of two edaphic ecotypes viz. erect and repent. While the former is a facultative calcicole, the latter is an obligate calcifuge. The present paper gives an account of plant performance and cytological behaviour of colchicine induced tetraploids that were produced by treating the inbred lines of these plants with 0.3% colchicine for 12 hours. Majority of the surviving plants were tetraploids exhibiting a slower growth delayed phenological events, deep green thick green rough foliage, increased dry matter production, enlarged epidermal and mesophyll cells, chloroplast, stomatal apparatus and frequency, pollen grains and a reduced seed viability than that of the diploids. While repent ecotype tetraploids had increased flower number per plant than that of the diploids, reverse was observed in the erect ecotype tetraploids.At metaphase I, uni, bi, tri, tetra and hexa-valents in a varying frequency were observed in the colchiploids and the total number of multivalent associations and univalents per PMC on an average was equal in the tetraploids of each ecotype. This indicates a competition for pairing between homologous chromosomes which is supported by the fact that a significant positive correlation between chiasma frequency and multivalent formation was also observed. However, the colchiploids of the erect ecotype possessed greater number of multivalents and univalents than those of the repent ecotype colchiploids. Further stages of meiosis in the colchi-ploids were abnormal and resembled the typical autoploid meiois. Multiple spindles, complement fractionation and secondary association were also observed. The contribution of univalents and multivalents in inducing pollen sterility in the tetraploid material is significant. Attempts to cross these colchiploids with diploids produced non-viable abortive seeds. Artificial crossings amongst these colchiploids resulted in an early degeneration of the ovary and a failure of the seed formation. Natural self pollination yielded 5-9% of seeds that died after an extrusion of radicle when germinated.The study indicated that the two edaphic ecological races (ecotypes) exhibit a differential susceptibility to colchicine in relation to frequency of induced polyploids, morphological and reproductive features, number and frequrncy of chromosomal associations and univalents, pollen and seed fertility. However, a general agreement in having a morphological gigas, slowed growth rate, delayed phenological events and reduced reproductive capacity occurs in the tetraploids in comparison to the diploid progenitors, of both the ecotypes. A tendency to change the plant form (repent erect), which is governed by a single gene, was not observed, indicating thereby that the gene remains uneffected by the drug, in both the ecological races of M. dianthera.