• Shanti Chandrashekaran

      Articles written in Journal of Genetics

    • RFLP approach to breeding for quantitative traits in plants—a critique

      V. Arunachalam Shanti Chandrashekaran

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      A number of recent papers suggest that use of RFLPs as markers offers a clear advantage in breeding for improvement in quantitative traits (QTs). The concepts underlying establishment of linkage between RFLP markers and QT loci stem from three papers on tomato published during 1987–1991. Essentially, continuously varying QT phenotypes are assigned to RFLP genotype classes, which can be considered to be determined by a single, diallelic gene with codominant alleles. Linkage is inferred through statistical analysis. Similarly, interaction between markers and QT is also tested by an analysis of variance. Here, the statistical methods employed in these three papers to detect linkage are critically evaluated, especially because subsequent investigations take the concepts developed in these papers as proven. In this paper, we examine the three fundamental papersde novo. We scrutinize the methods employed and the inferences drawn to bring to light what we believe are conceptual drawbacks.

    • Effects of mutations at thestambh A locus ofDrosophila melanogaster

      M. Kumar Minu Joseph Shanti Chandrashekaran

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      We report novel findings on the cytogenetic location, functional complexity and maternal and germline roles of thestambh A locus ofDrosophila melanogaster. stmA is localized to polytene bands 44D1.2 on 2R.stmA mutations are of two types: temperature-sensitive (ts) adult and larval paralytic or unconditional embryonic or larval lethal. Twelve alleles reported in this study fall into two intragenic complementing groups suggesting thatstmA is a complex locus with more than one functional domain. Some unconditional embryonic lethal alleles show a ‘neurogenic’ phenotype of cuticle loss accompanied by neural hypertrophy. It is shown that embryos of ts paralytic alleles also show mild neural hypertrophy at permissive temperatures while short exposure to heat induces severe cuticle loss in these embryos.stmA exerts a maternal influence over heat-induced cuticle loss. Unconditional embryonic lethal alleles ofstmA are also germline lethal.

    • Meiosis in a triploid hybrid of Gossypium: high frequency of secondary bipolar spindles at metaphase II

      Mosareza Vafaie-Tabar Shanti Chandrashekaran

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      Studies on meiosis in pollen mother cells (PMCs) of a triploid interspecific hybrid ($3x = 39$ chromosomes, AAD) between tetraploid Gossypium hirsutum ($4n = 2x = 52$,AADD) and diploid G. arboreum ($2n = 2x = 26$,AA) are reported. During meiotic metaphase I, 13 AA bivalents and 13 D univalents are expected in the hybrid. However, only 28% of the PMCs had this expected configuration. The rest of the PMCs had between 8 and 12 bivalents and between 12 and 17 univalents. Univalents lagged at anaphase I, and at metaphase II one or a group of univalents remained scattered in the cytoplasm and failed to assemble at a single metaphase plate. Primary bipolar spindles organized around the bivalents and multivalents. In addition to the primary spindle, several secondary and smaller bipolar spindles organized themselves around individual univalents and groups of univalents. Almost all (97%) of the PMCs showed secondary spindles. Each spindle functioned independently and despite their multiple numbers in a cell, meiosis I proceeded normally, with polyad formation. These observations strongly support the view that in plant meiocytes bilateral kinetochore symmetry is not required for establishing a bipolar spindle and that single unpaired chromosomes can initiate and stabilize the formation of a functional bipolar spindle.

    • Cytotaxonomical analysis of Momordica L. (Cucurbitaceae) species of Indian occurrence

      L. K. Bharathi A. D. Munshi Vinod Shanti Chandrashekaran T. K. Behera A. B. Das K. Joseph John Vishalnath

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      Somatic chromosome number and detailed karyotype analysis were carried out in six Indian Momordica species viz. M. balsamina, M. charantia, M. cochinchinensis, M. dioica, M. sahyadrica and M. cymbalaria (syn. Luffa cymbalaria; a taxon of controversial taxonomic identity). The somatic chromosome number $2n = 22$ was reconfirmed in monoecious species (M. balsamina and M. charantia). Out of four dioecious species, the chromosome number was reconfirmed in M. cochinchinensis $(2n = 28)$, M. dioica $(2n = 28)$ and M. subangulata subsp. renigera $(2n = 56)$, while in M. sahyadrica $(2n = 28)$ somatic chromosome number was reported for the first time. A new chromosome number of $2n = 18$ was reported in M. cymbalaria against its previous reports of $2n = 16$, 22. The karyotype analysis of all the species revealed significant numerical and structural variations of chromosomes. It was possible to distinguish chromosomes of M. cymbalaria from other Momordica species and also between monoecious and dioecious taxa of the genus. Morphology and crossability among the dioecious species was also studied. Evidence from morphology, crossability, pollen viability and chromosome synapsis suggests a segmental allopolyploid origin for M. subangulata subsp. renigera. The taxonomic status of the controversial taxon M. cymbalaria was also discussed using morphological, karyological and crossability data.

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