Unexpected pattern of pearl millet genetic diversity among ethno-linguistic groups in the Lake Chad Basin.

Despite of a growing interest in considering the role of sociological factors in seed exchanges and their consequences on the evolutionary dynamics of agro-biodiversity, very few studies assessed the link between ethno-linguistic diversity and genetic diversity patterns in small-holder farming systems. This is key for optimal improvement and conservation of crop genetic resources. Here, we investigated genetic diversity at 17 SSR markers of pearl millet landraces (varieties named by farmers) in the Lake Chad Basin. 69 pearl millet populations, representing 27 landraces collected in eight ethno-linguistic farmer groups, were analyzed. We found that the farmers' local taxonomy was not a good proxy for population's genetic differentiation as previously shown at smaller scales. Our results show the existence of a genetic structure of pearl millet mainly associated with ethno-linguistic diversity in the western side of the lake Chad. It suggests there is a limit to gene flow between landraces grown by different ethno-linguistic groups. This result was rather unexpected, because of the highly outcrossing mating system of pearl millet, the high density of pearl millet fields all along the green belt of this Sahelian area and the fact that seed exchanges among ethno-linguistic groups are known to occur. In the eastern side of the Lake, the pattern of genetic diversity suggests a larger efficient circulation of pearl millet genes between ethno-linguistic groups that are less numerous, spatially intermixed and, for some of them, more prone to exogamy. Finally, other historical and environmental factors which may contribute to the observed diversity patterns are discussed.

Characterization of two members of the Arabidopsis thaliana gene family, At beta fruct3 and At beta fruct4, coding for vacuolar invertases.

We have isolated and characterized two Arabidopsis thaliana cDNAs and their cognate genes, At beta fruct3 and At beta fruct4, encoding vacuolar forms of invertase. Our sequencing results showed that the gene At beta fruct3 is located downstream of the 3-ketoacyl-acyl carrier protein synthase III gene (AtKasIII). At beta fruct3 and 4 are functional and organized into seven exons and six introns with an identical organization. The At beta fruct3 and At beta fruct4 genes encode, respectively, polypeptides of 648 and 664 residues that contain all the characteristic hallmarks of vacuolar invertases. A. thaliana is the first plant of which both cell-wall (At beta fruct1 and At beta fruct2) and vacuolar (At beta fruct3 and At beta fruct4) genes are characterized. The same number of exons and introns is seen in the genes At beta fruct1, At beta fruct3 and At beta fruct4 as well as in all other invertase genes described to date. However, the position of the third intron is different in At beta fruct3 and At beta fruct4. At beta fruct2 shows a different organization. A neighbour-joining distance tree shows that the A. thaliana vacuolar invertases described here are, as expected, more closely related to vacuolar invertases from other plant species (e.g., carrot) than to the A. thaliana cell-wall invertases. The evolution of plant invertase genes from a common ancestral gene is discussed. Our results demonstrate that in A. thaliana, at least two genes encoding vacuolar invertases are expressed during the development of the plant. Southern blot hybridization experiments suggest the presence of one copy of, respectively, At beta fruct3 and At beta fruct4 per haploid genome, and Northern blot analysis demonstrates that vacuolar invertase genes are highly expressed in stems, roots, flowers and at very low levels in mature leaves.

The wheat mitochondrial rps13 gene: RNA editing and co-transcription with the atp6 gene.

Northern analyses and reverse transcription-polymerase chain-reaction (RT-PCR) experiments, followed by PCR amplification product sequencing, were performed on total mitochondrial (mt) RNAs from wheat seedlings and tissue cultures. It was shown that the rps13 gene, which encodes ribosomal protein S13, and the atp6 gene, which encodes subunit 6 of the ATP synthase complex, were co-transcribed. However, rps13 transcripts were virtually undetectable in seedlings under conditions where atp6 transcripts appeared abundant. In addition, markedly higher steady state transcript levels were observed in tissue culture. Expression of the mitochondrial rps13 gene was confirmed by showing that its transcripts were edited. Slight differences between editing patterns of tissue-culture and whole-plant transcripts were found. Taken together, these results suggest that in vitro culture could disturb the post-transcriptional regulation of gene expression.

Genes encoding subunit 6 of NADH dehydrogenase and subunit 6 of ATP synthase are co-transcribed in maize mitochondria.

The mitochondrial nad6 gene of maize was identified and mapped 1 kb downstream from the atp6 gene. It encodes a 220 amino-acid polypeptide. Using Northern hybridization experiments and RT-PCR analysis, we showed that both nad6 and atp6 are co-transcribed in maize mitochondria. RNA editing of the mitochondrial nad6 transcript was studied by cDNA sequencing. Twelve edited sites were identified at the same positions as those already identified in the wheat mitochondria nad6 transcript. Alignments of nucleotide and amino-acid sequences of the mitochondrial nad6 genes of maize, wheat, and Brassica campestris, show that the wheat gene encodes a shorter polypeptide (229 amino acids) than was previously thought.