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A pair of genes
Readers of this journal know very well that genome sizes vary enormously between different species, based mainly on large differences in the amount of non-coding DNA sequences. However, there is also some variation, albeit relatively small, between species in respect of gene numbers. Why then does one species possess, and by implication need, 10 % more genes than another? The conundrum is beautifully illustrated by the work of Rondeau et al. (La Réunion and Lausanne, France, pp. 1307–1314) on the evolution of the gene(s) encoding plastid NADP-dependent malate dehydrogenase (NADP-MDH). This enzyme is involved in shuttling malate out of the plastid but in C4 plants it also has a key role in the initial fixation of CO2. The question thus arises as to whether C4 plants have two separate genes encoding NADP-MDH or whether both functions are covered by one gene. The authors have used RT–PCR to amplify the NADP-MDH sequences from a number of species in the sub-family Panicoideae of the family Poaceae. This sub-family contains both C3 and C4 members. Phylogenetic analysis of the sequence data show a number of features, of which we focus here on the existence of two forms of the gene: NMDH-I and NMDH-II. These appear to have arisen by duplication of an ancestral gene followed by sequence divergence. Does the possession of two genes relate to the ability to carry out C4 photosynthesis? Certainly several C4 species possess and express both NMDH-I and II while the C3 Oryza sativa possesses only a NMDH-II-like gene. However, this neat story is spoiled by a classic C4 species, Zea mays, which also possesses only a NMDH-II-like gene. So, within the same sub-family there are species that possess two genes covering two roles and at least one species that possesses only one gene to cover the same two roles. Functional genomics can thus reveal some very intriguing situations.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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