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temporal requirement for both Pdgfaa signal-ing and Mirn140 action, although how this would be achieved is not clear. Alternatively, there may a critical threshold of Pdgfra signal-ing required for normal NCC migration, with either too little or too much signaling leading to aberrant migratory behavior. Regardless of the mechanism, as migration of other cranial NCC populations seems normal following knockdown of mirn140, there clearly exists an exquisite specificity for Pdgfra action on migrating palatal precursor cells. This work also establishes the exciting possibility that other miRNAs may contribute to the migra-tion of other subsets of NCCs.

The development of the primary and sec-ondary palates in humans and mice is com-prised of distinct morphogenetic events12. As fish do not have a nasopharynx, they natu-rally lack a secondary palate. Nevertheless, it is quite possible that primary and second-ary palatogenesis may be regulated through conserved mechanisms, the identification of which may be aided by further analysis of zebrafish palatal mutants. In addition, the exciting demonstration of Mirn140 func-tion underscores the likely significant role of miRNAs in orofacial development.

1. Spritz, R.A. Curr. Opin. Pediatr. 13, 556–560 (2001).

2. Schutte, B.C. & Murray, J.C. Hum. Mol. Genet. 8, 1853–1859 (1999).

3. Eberhart, J.K. et al. Nat. Genet. 40, 290–298 (2008).4. Conrad, R., Barrier, M. & Ford, L.P. Birth Defects Res.

78 (Part C), 107–117 (2006).5. Eulalio, A., Huntzinger, E. & Izaurralde, E. Cell 132,

9–14 (2008).6. Betsholtz, C. Birth Defects Res. 69 (Part C), 272–285

(2003).7. Tallquist, M.D., Weismann, K.E., Hellstrom, M. &

Soriano, P. Development 127, 5059–5070 (2000).8. Soriano, P. Development 124, 2691–2700 (1997).9. Pickett, E.A., Olsen, G.S. & Tallquist, M.D. Development

135, 589–598 (2008).10. Xu, X., Bringas Jr., P., Soriano, P. & Chai, Y. Dev. Dyn.

232, 75–84 (2005).11. Tallquist, M.D. & Soriano, P. Development 130, 507–

518 (2003).12. Chai, Y. & Maxson, R.E., Jr. Dev. Dyn. 235, 2353–2375

(2006).

Plant breeders go back to natureDani Zamir

A new mapping study identifies a natural protein variant influencing oil content and composition in maize seeds. This example illustrates the power of exploiting natural variation to improve crop yields and address current breeding challenges.

Dani Zamir is at the Faculty of Agriculture of The Hebrew University of Jerusalem, Rehovot 76100, Israel. e-mail: zamir@agri.huji.ac.il

Plant breeders are currently faced with the chal-lenge of bringing about a new ‘Green Revolution’ to feed the ever-growing world population and to develop alternative energy crops1. Indeed, breeding responsibilities have become multi-faceted. Crops must continue to provide the basic caloric needs and dietary micronutrients required for human health, yet global climate changes demand that modern varieties be re-adapted to changing temperatures and rainfall restrictions. There has also been a recent push to develop bioenergy crops, characterized by efficient biomass production for use as biofuels. Thus, the power to breed the best new varieties must be based on a broad platform of genetic diversity, in which up-to-date genomic technol-ogies can tap newly identified genes and alleles for trait improvement. On page 367 of this issue, Zheng et al.2 take a step towards meeting today’s breeding challenges by combining modern plant breeding tools with nature’s rich biodiversity to isolate a key gene that can be used to regulate oil content and composition in maize.

Natural diversity and yieldMaize, one of the founding models of quan-titative variation and a major crop plant, has been exploited recently in genetic association

analyses to identify signatures of selection for quantitative trait loci (QTLs) implicated in crop domestication3. Although these genes affect diverse developmental phenotypes, few have obvious and immediate potential to benefit breeding goals. Now, Zheng et al.

report the union of classical map-based clon-ing of a QTL with a species-wide association analysis to reveal the molecular nature of a gene that modifies an important yield com-ponent of maize—oil and oleic-acid content in seeds. The authors first completed a suc-cessful map-based cloning of a diacylglycerol

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acyltransferase (DGAT) that catalyzes the final step in the triglycerol biosynthetic pathway. They then used ultra high–resolution genetic mapping to delimit the effect to an insertion of a single amino acid. To verify the functional significance of this change, the authors car-ried out key transgenic experiments in plants, and showed that the high-oil allele produced a protein with increased enzymatic activity fol-lowing ectopic expression in yeast. To cap it all off, the authors surveyed DGAT variation in a panel of 71 maize lines differing in oil content and showed that the high-oil allele is present in maize wild relatives and was subsequently lost, either during early domestication or during the breeding of modern-day varieties. Not only is it rare to find such a comprehensive analysis of a QTL, but these results also provide a route for improving maize oil content and for modify-ing oil composition in other plant species that might be important for biofuel production.

The good news for modern breeding is that Zheng et al. are not alone in their efforts. A recent study4 has reported on the value of genomic analysis of maize biodiversity to achieve the breeding goal of combating human dietary vitamin A deficiency. This essential micronutrient is synthesized via the well-characterized carotenoid biosyn-thetic pathway; thus, the genes encoding the carotenoid-forming enzymes were obvious QTL candidates. An association analysis on a panel of 282 inbreds phenotyped for carotenoid composition enabled the identi-fication of lycopene epsilon cyclase (lycE) as a gene responsible for increasing the flux of β-carotene, or pro–vitamin A. QTL linkage data and an elegant confirmatory mutant

analysis, integrated with the association mapping, showed that the high pro–vitamin A content is due to elevated expression of lycE. To enhance the rate of breeding for the high vitamin phenotype, this study identified new genetic sources in the maize core collec-tion that were enriched for the compound. The authors then took a step further and developed simple PCR assays for molecular markers that now enable breeders in devel-oping countries to more efficiently produce maize grains with higher pro–vitamin A concentrations.

Tomato provides yet another example of the indispensable value of natural biodiver-sity for yield improvement in a crop that har-bors much lower genetic diversity compared to maize. In one example, chromosome seg-ments from an inedible green-fruited wild species were introduced into the genetic background of an elite variety via marker-assisted selection5. This population enabled the identification of multiple yield-associated QTLs, one of which (Brix9-2-5) was delim-ited to a single base-pair change in an apo-plastic invertase coding sequence, where the wild-species allele increased the affinity of the enzyme to sucrose and increased sugar yield of tomatoes. The processing tomato hybrid AB2 harbors QTL from the Peruvian species and is presently a leading ketchup variety.

The way forwardUntil recently, it was thought that “there are fewer options available than previously to address current problems through traditional breeding techniques”6 and that genetic modi-fication technologies would largely replace

classical breeding. The science of plant breed-ing is still waking up from this transgenic dream. Although genetic modification tech-nologies have proven to be very powerful for introducing single gene traits (for example, resistances to insects and herbicides), the success rate for more complex traits, deter-mined by numerous interacting genes, is much lower. The age of ‘omics’ has identified many sequences that correlate with pheno-typic changes; however, our understanding of the networks of interactions that are taking place in vivo—in the field—is still lacking. As a result, our ability to make wise predictions about which candidate genes to transform, under which promoters to transform them, and whether to up- or downregulate them is still very limited.

On the other hand, natural biodiversity resources, in the form of wild species or old varieties, represent genotypes that were selected over evolutionary time for adapta-tion to natural environments and agricultural fields. The identification of QTL alleles that improve agricultural production allows breed-ers to introgress the traits into modern genetic backgrounds and to explore the molecular mechanisms that regulate these effects. The examples above show that the time has come for the plant breeding community to move back to nature.

1. McCouch, S. PLoS Biol. 2, e347 (2004).2. Zheng, P. et al. Nat. Genet. 40, 367–372 (2008). 3. Doebley, J.F., Gaut, B.S. & Smith, B.D. Cell 127,

1309–1321 (2006).4. Harjes, C.E. et al. Science 319, 330–333 (2008).5. Lippman, Z.B., Semel, Y. & Zamir, D. Curr. Opin.

Genet. Dev. 17, 545–552 (2007).6. Zamir, D. Nat. Rev. Genet. 2, 983–989 (2001).

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