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The phenotype reverts to normal (partly) if the level
of calcium in the cells is decreased. It turns out that apart from this defect,
a deletion of trishanku- as we have named the gene - causes early aggregates to fragment. Later,
the differentiated state of pre-stalk and pre-spore cells in the slug becomes
unstable. Nameeta Mujumdar is studying the expression of genes of interest in
the trishanku background, monitoring intercellular adhesion in the mutant
and looking for putative interacting partners of the trishanku gene product.
Trupti Kawli used a different approach to probe the cell
cycle-calcium link. She tried to complement known cell cycle mutations in the
yeast Saccharomyces cerevisiae with Dictyostelium cDNA and
discovered that a gene that encodes a small ribosomal protein, S4, is able to
rescue the cell cycle defect of the yeast cdc24-4 mutation. S4 appears to
be essential for survival in D. discoideum, but a partial abolition of
its function (via an antisense RNA construct) permits cell survival. These
‘antisense’ cells aggregate normally but their development is aberrant
thereafter: single aggregates give rise to multiple tips and many abortive
initiations of fruiting body formation (Figure 3). Smita Amarnath is
characterizing the regulatory regions of the S4 gene in order to understand
better the basis behind the rescue of cdc24-4 by S4 DNA. Ritwick
Sawarkar is exploring the possibility of establishing pre- and post-aggregation
differences in mitochondrial status as yet another functional heterogeneity
between amoebae. He is trying to see whether this might help in understanding a
curious observation made earlier by
R. Baskar, namely that the
heterogeneity might be manifested by spores too and thereby run across
generations. |
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Differentiation in the cellular slime moulds poses an
evolutionary problem. Because the amoebae that contribute to the stalk
die, they can not pass on their genes to the next generation. That being
the case, why do they aggregate with the others at all? As we have seen,
they co-aggregate and form a structure whose purpose appears to be the
efficient dispersal of spores – they behave in an altruistic fashion.
Altruism is a fascinating issue in evolutionary biology and two general
explanations have been advanced for explaining its presence. One, the behaviour
may represent a group adaptation – in the long run, it may be sufficiently ‘good
for the group’ to outweigh its disadvantage to the individual. The closer the
kinship or degree of relatedness between the members of a group, the more
attractive this explanation becomes. Two, though a cell that joins an aggregate
has a finite chance of dying, it may do better than it would by remaining alone.
Sonia Kaushik explored these alternatives by observing the behaviour of amoebae
of one genotype when mixed with those of another. She found that there was a
strong tendency for cells belonging to different genotypes to become segregated
from each other, favouring the idea that amoebae preferred to associate with
their kin. On the other hand, kinship per se was a poor predictor of the degree
of altruistic behaviour that was exhibited. Her work is being extended along
three different lines. Bandhana Katoch is continuing the mixing experiments and
is making explicit counts of spore and stalk cells in genetic chimaeras. Saby
John is following up Sonia’s observation that a family of lipid-soluble toxic
compounds, collectively known as DIF, may be used by the stronger cells in an
aggregate to subdue the weaker ones and force them to die. Channabasavan Gowda
has set up genetic crosses in D. giganteum and is making use of
recombinants to test explicitly whether amoebae can effectively distinguish
between members of their own genotype (with whom they share 100% of their genes)
and recombinant progeny (with whom they share, on average, 50% of their genes).
A. Srinivasa Rao works independently on statistical
analyses of epidemiological problems, especially AIDs. Apart from
experimental work, we are also investigating theoretical models for the
evolution of phenotypic plasticity and dominance.
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Publications
Azhar, M., Krefft, M.,
Saran, S., Weeks, G. and Nanjundiah, V. (1998)"Calcium levels correlate with
cell cycle phase and affect the level of the cyclin B transcript in
Dictyostelium discoideum". FEMS Microbiology Letters, 161, 193-199.
Baskar, R., Chhabra, P., Mascarenhas, P. and Nanjundiah, V. (2000) "A cell
type-specific effect of calcium in pattern formation and differentiation
in Dictyostelium discoideum", Int. J. Dev. Biol. 44, 491-498.
Kawli,
T. and Kaushik, S. (2001) “Cell
fate choice and social evolution in
Dictyostelium discoideum:
Interplay of morphogens and heterogeneities”. J.
Biosci.
26, 130–133.
Kawli, T., Venkatesh, B.
R., Kennady, P. K., Pande, G. and
Nanjundiah, V. (2002) “Correlates of developmental cell death in
Dictyostelium discoideum”. Differentiation 70, 272-281.
Nanjundiah, V. (2003)
"Phenotypic Plasticity and Evolution by Genetic Assimilation" (in
Origins of Organismal Form, G. Müller and S. A. Newman, eds., MIT Press, pp
244-263)
Popular articles
Nanjundiah, V. (1999) "Delbrück's Publications in Biology", Resonance (November)
35-53.
Uwins, P
and Nanjundiah, V. (2000)"How Small Can You Get?", Chemistry in Australia, 67
(4), 12-14.
Nanjundiah, V. “Alan Turing and “The Chemical Basis of Morphogenesis” (pp 33-44,
in Morphogenesis and Pattern Formation in Biological Systems, T. Sekimura, ed., Springer-Verlag, Tokyo, 2003).
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