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Meiotic products in higher plants should undergo a determined number of mitotic cycles before differentiating gametes. This creates a unique meiosis-mitosis interface, traverse of which is an absolute requirement for gametophyte development. In the absence of cytokinesis during megasporogenesis - as seen in the bisporic and tetrasporic types - the haploid nuclei produced by meiosis are driven to undergo mitotic cycles within the same cell. Similarly, the last of the mitotic cycles leads to a unique type of cell wall formation resulting in cellularization of the coenocytic female gametophyte, creating a mitosis-cellularization interface. Cell cycle regulation in terms of the molecules that interface with these two key spatio-temporal developmental settings should be of interest to both cell and developmental biologists. High throughput techniques of functional genomics are required for both interpretation of female gametophyte evolution and success of the biotechnological initiatives of transferring apomixis-related genes to crop plants.

Electronmicroscopic study has revealed a dicentric chromosome in the root tip cells of diploid Aloe vera, hitherto unreported in somatic cells of plants. The morphological aspects are described and the implications of such structural features of the dicentric are addressed in a functional perspective. It is also proposed that the presence of two functional kinetochores need not invariably lead to chromosome instability and loss.

The underlying mechanisms responsible for elimination of three of the four meiotic products during microsporogenesis in sedges have remained obscure. Although programmed cell death (PCD) is known to be an integral part of plant development, several aspects, such as the enormous size difference between functional and eliminated cells, nuclear viability of the smaller cells targetted for elimination and the possible dangers of employing PCD-like mechanisms during gametophyte development point to a novel way of selective cell elimination associated with asymmetric division. This paper throws some light on the possible roles of cell plate dynamics and cell fate determinants in pseudomonad development. The rarity and significance of asymmetric division with morphogenetic consequences associated with meiosis are discussed in the context of generally prevalent, mitosis- associated asymmetric divisions during development.

The permanent translocation heterozygote, Rhoeo spathacea shows a characteristic ring of all chromosomes at metaphase-I of male meiosis. Elimination of bivalent formation completely due to extreme heterozygosity poses a special problem for reductional division at meiosis-I where the maternal and the paternal sets need to be separated to different poles. Unlike bivalent meiosis, the functions of SC formation, pairing and recombination required for reductional segregation have to be fulfilled by only telomeres which are the only areas of contact among the chromosomes in the ring. The participation of telomeres in these events assumes significance in view of the balanced lethal gene system imposed by translocation heterozygosity, and the particular requirement that only alternate chromosomes segregated to the same pole results in fertile pollen. This paper throws some light on the telomere functions in R. spathacea in the light of our present understanding of telomere biology. The prospects of using R. spathacea as a model organism to dissect molecular aspects of several telomere related functions are also discussed.

Karyotypes of six species of Barleria L. occurring in South India viz., B. acuminata, B. buxifolia, B. gibsoni, B. involucrata var. elata, B. noctiflora and B. strigosa are studied. All the six species have the same chromosome number of 2n=40. However, each species is distinct in respect of relative positions of centromere, presence/absence and position of satellites and total haploid chromatin length. The diagnostic value of the karyomorphological details in these taxa is discussed.

A central question in biology is how cell fate is specified during development of a multicellular organism. Flowering plants use two major pathways of asymmetric cell divisions in a spatio-temporal manner to achieve required cellular differentiation. In the 'one mother--two different daughters' pathway, a mother cell mitotically divides to produce two daughter cells of different size and fate. By contrast, the 'coenocyte-cellularization' pathway involves formation of a coenocyte, nuclear migration to specific locations of the coenocyte and cellularization of these nuclei by unique wall forming processes. Given that cell fate determinants play a key role in establishing cell identity, their allocation to daughter cells in the two pathways needs to be understood in terms of the unique cell cycle regulatory mechanisms involved. Most of the information available on cell fate determination in flowering plants is in the form of genes identified from mutant analysis. Novel techniques of interrogating individual plant cells in vivo are necessary to advance the extant knowledge from genetics to functional genomics data bases.

A central question in biology is how cell fate is specified during development of a multicellular organism. Flowering plants use two major pathways of asymmetric cell divisions in a spatio-temporal manner to achieve required cellular differentiation. In the 'one mother--two different daughters' pathway, a mother cell mitotically divides to produce two daughter cells of different size and fate. By contrast, the 'coenocyte-cellularization' pathway involves formation of a coenocyte, nuclear migration to specific locations of the coenocyte and cellularization of these nuclei by unique wall forming processes. Given that cell fate determinants play a key role in establishing cell identity, their allocation to daughter cells in the two pathways needs to be understood in terms of the unique cell cycle regulatory mechanisms involved. Most of the information available on cell fate determination in flowering plants is in the form of genes identified from mutant analysis. Novel techniques of interrogating individual plant cells in vivo are necessary to advance the extant knowledge from genetics to functional genomics data bases.

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.

Programmed cell death (PCD) is an integral part of both animal and plant development. In animals, model systems such as Caenorhabditis elegans, Drosophila melanogaster, and mice have shown a general cell death profile of induction, caspase mediation, cell death, and phagocytosis. Tremendous strides have been made in cell death research in animals in the past decade. The ordering of the C. elegans genes Ced-3, 4 and 9, identification of caspase-activated DNase that degrades nuclear DNA during PCD, identification of signal transduction modules involving caspases as well as the caspase-independent pathway, and the involvement of mitochondria are some of the findings of immense value in understanding animal PCDs. Similarly, the caspase inactivation mechanisms of infecting viruses to stall host cell death give a new dimension to the viral infection process. However, plant cell death profiles provide an entirely different scenario. The presence of a cell wall that cannot be phagocytosed, absence of the hallmarks of animal PCDs such as DNA laddering, formation of apoptotic bodies, a cell-death-specific nuclease, a biochemical machinery of killer enzymes such as caspases all point to novel ways of cell elimination. Large gaps in our understanding of plant cell death have prompted speculative inferences and comparisons with animal cell death mechanisms. This paper deals with both animals and plants for a holistic view on cell death in eukaryotes.

Two interspecific F1 hybrids of Citrus species were examined for inheritance of leaf phenolic compounds. Both the hybrids revealed a reduced number of compounds compared with those of the parents. In addition, the spot pattern of each taxon offered a basis for analysis of inheritance pattern. Due to the production of clones with a consequent reduction in the number of sexual offspring by nucellar embryony in these plants, the unusual pattern of inheritance observed in the hybrids could not be categorized as either a product of disturbed Mendelian ratios or as transgressive segregation affecting only a few individuals in the segregating generations.