**1. Introduction**

Gene duplication is one of the key driving forces in the evolution of genes and important features of genomic architecture of living organisms including plants. Moreover, much of the plant diversity may have arisen largely due to duplication, followed by divergence and adaptive specialization of the pre existing genes (Ohno,1970; Zhang, 2003; Flagel & Wendel,2009). Current impetus on genomic sequence data provides substantial evidence for the profusion of duplicated genes in all organisms surveyed. Functional divergence after gene duplication can possibly result in two alternative evolutionary fates: i) neofunctionalization where one copy acquires an entirely new function whereas the other copy maintains the original function. ii) Subfunctionalization, in which each copy adopts part of the task of their parental gene (Ohno,1970; Nowak et al., 1997; Jenesen,1976; Orgel,1977;Hughes,1994). However, subfunctionalization is reported as a more prevalent outcome than neofunctionalization in nature. In any case, functional divergence of such paralogous proteins is found to be the key force shaping molecular network in organisms (Ohno, 1970). Recent studies also suggest that duplicate genes diverge mostly through the partitioning of gene expression as in subfunctionalization (Force et al.,1999; Wagner,2000; Gu et al.,2002). In addition, subfunctionalization can also take place at the protein function level leading to functional specialization, when one of the duplicated genes becomes better at performing one of the original functions of the progenitor gene (Hughes, 1994; Gu et al.,2002; Conant &Wolfe, 2008; Hughes, 1999; Zhang et al., 2002).

Myo-inositol-1-phosphate synthase (MIPS;EC5.5.1.4) is an evolutionary conserved enzyme which catalyzes the rate limiting step in well conserved inositol biosynthetic pathway and is extremely widespread in living organisms including plants (Loewus & Murthy, 2000; Majumder et al., 2003). The evolution of MIPS gene/ protein among the prokaryotes seems to be more divergent and complex than amongst the eukaryotes, however they preserve a conserve core catalytic domain among the MIPS proteins (Majumder et al., 2003).

Many of the plant species are known to contain more than one copy of gene encoding MIPS and are hypothesized to arise through gene duplication. Expression studies of multiple gene encoding MIPS have revealed the possibility of specialized role for individual enzyme isoforms. Previously, two genes encoding MIPS have been identified and characterized from chickpea by Kaur *et al.* A comparative study of two divergent genes (*CaMIPS1* & *CaMIPS2*)

L- *Myo*-Inositol 1-Phosphate Synthase

(Fig-2 & 3).

through gene duplication in course of time.

over time (Fig-3) (Stein & Geiger, 2002; Norman et al.,2002).

catalytic domain of the enzyme. (Stein & Geiger, 2002)

(MIPS) in Chickpea: Gene Duplication and Functional Divergence 227

inositol and its derivatives proliferated dramatically. So far, inositol and its derivatives have been shown to be involved in growth regulation, membrane biogenesis, hormone regulation, signal transduction, pathogen resistance and stress adaptation in higher plants

Since the usage and distribution of inositol and inositol derivatives are reported in all domains of life, it is imperative to contain MIPS enzyme in diverse organisms such as archaea, eubacteria, parasites, animals, higher plants and many others. Few higher plants and algae are reported to have both cytosolic and chloroplastic isoforms of MIPS. However, the biochemical and enzymatic properties of these two forms do not differ significantly between each other. Recent studies suggest that rice chloroplastic MIPS is coded by *OsINO1-*

The structural gene coding (*INO1*) for this ancient enzyme was first identified and cloned in *Saccharomyces cerevisiae* (Donahue & Henry,1981). Subsequently more than 80 *INO1* genes

Evolution and diversification of MIPS has been highlighted by Majumder et al. (2003) and a clear difference between prokaryotic and eukaryotic MIPS protein sequences was observed when compared among each other. The MIPS protein sequences of prokaryotes are quite divergent among themselves and significantly distinct than any other known eukaryotic sequences. In contrast, the eukaryotic MIPS sequences show remarkable similarities among each other. A phylogenetic tree constructed to include few representative MIPS sequences from diverse organisms present an overall evolutionary divergence of this enzyme in the biological kingdom. The higher plants constitute one close subgroup, while the higher animals, protozoa, fungi form the other subgroups in the eukaryotic cluster (Fig-2). In Archeoglobus, MIPS shows more sequences similarity to the eukaryotic MIPS than the other known prokaryotic ones and thereby all eukaryotic MIPS seems to have evolved from one common stock, probably from the fusion of an archaebacterial and eubacterial MIPS genes

Four stretches of amino acid residues (GWGGNG, LWTANTERY, NGSPQNTFVPGL and SYNHLGNNDG) are found to be conserved in MIPS proteins of all eukaryotes and among them; SYNHLGNNDG is identified as highly conserved. Interestingly among higher plants, MIPS enzyme shows greater conservation in addition to these four domains (Fig-3). Many of the plant species also possess multiple genes encoding MIPS and are thought to arise

Subsequent analysis of crystal structures of various MIPS proteins provide ample evidence towards the presence of conserved "core structure" in all MIPS proteins throughout evolution. Moreover, some of the important amino acid residues are identified in the active site of the yeast MIPS and are shown to be highly conserved in all eukaryotic MIPS. These amino acids are considered to be the part of a "eukaryotic core structure" which has remained largely the same during evolution, despite the divergence in rest of the sequences

Crystal structure analysis of MIPS from *Saccharomyces cerevisiae* also revealed that each monomer of the homo-tetrameric MIPS has three functionally important structural domains namely the NAD binding Rossman fold, the catalytic binding site and the core domain. This study also exemplifies a case of induced fit model for binding of the substrate with the

(Loewus & Murthy, 2000; Stevenson et al., 2000; Michell,2008).

*1* gene located on chromosome 3 (RayChaudhuri et al.1997;Ray et al., 2010).

were reported from various sources including both prokaryotes and eukaryotes.

reveals features of both functional redundancy and diversification (Kaur et al.*,* 2008). This chapter explores how a possible gene duplication of MIPS gene in chickpea lead to a functional diversification that perhaps contributed adaptive evolution to the plant.
