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alternatively, may have arisen by convergent evolution, as mimicry of host
proteins.
20.7
CTLD cps in Genomes of Pre - Metazoans and Plants
Early fi ndings of CTLD sequences in the genome of the fl owering plant
Arabidopsis
thaliana
, with a well-conserved ortholog for one of the putative CTLDcps also
present in the rice genome, were very perplexing [2]. However, very recent reports
of more numerous CTLDcps in genome sequences of a pre- Metazoan, the cho-
anofl agellate
Monosiga brevicollis
(12 CTLDcps) [9], and the unicellular green alga
Chlamydomonas reinhardtii
[10] require a serious reconsideration of the Metazoan
hypothesis.
The statistics of the
C. reinhardtii
analysis are compelling, being reminiscent of
the general conclusions from the invertebrate and vertebrate genomes we have
already outlined. They strongly argue that these CTLDcps have evolved within
Chlamydomonas
for specifi c roles. Thus, of the 11 CTLDcps containing a total of
67 CTLDs, seven also contain scavenger-receptor cysteine-rich (SRCR) domains,
a combination rarely seen in Metazoans and not in CTLDcps for worm, fl y and
human shown in Figure 20.2. Also, 18 of the CTLDs possess the conserved
sequence motifs (
E
P
N
/
Q
P
D
and W
N
D) associated with Ca
2+
- dependent carbo-
hydrate binding.
How can the presence of this novel, and likely functional, collection of CTLDcps
in a distant non-Metazoan branch be explained? Although the details of the branch-
ing pattern around the time of the Metazoan 'explosion' are still an ongoing debate
(please see Figure 20.5), such ambiguity cannot explain the apparent absence of
CTLDs in fungi, but variable abundance in plants and algae. Although Wheeler
et
al.
[10] interpret their fi ndings in terms of loss of CTLDcp genes or divergence of
the CTLD beyond recognition as the eukaryotes evolved, a more plausible explana-
tion may be that the plant/algae branch acquired primordial CTLD genes by hori-
zontal transfer, possibly multiple times. Under this model, the patterns of CTLDcps
in subbranches such as
Chlamydomonas
simply represent evolution starting at
punctuated times to happily build anew groups of CTLDcps to perform novel
needed roles, using one of its best domain building blocks - the structurally fl exible
and functionally versatile CTLD. This explanation is more consistent with the
knowledge we have acquired in this chapter of the CTLD as an evolutionarily very
successful domain in Metazoans as it does not require an explanation as to why
the CTLD was not equally successful in fungi and plants!
20.8
Conclusions
In this chapter we have taken a historical approach to showing how understanding
of the structure, function and evolution of C-type lectins has unfolded. We have
