Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Quantitative Trait Loci and Single Nucleotide Polymorphism Cytogenet Genome Res 117:305–312 (2007) DOI: 10.1159/000103192 Parent-of-origin specific QTL – a possibility towards understanding reciprocal effects in chicken and the origin of imprinting M. Tuiskula-Haavisto J. Vilkki Biotechnology and Food Research, MTT Agrifood Research Finland, Jokioinen (Finland) lecular organization and expression data of the most prom- ising candidate chromosome regions to show monoallelic expression in chicken. Intensive progress in poultry breeding began about 60 years ago with pure line breeding. The idea of crossing in- bred lines to exploit heterosis came from corn breeding, where it was successfully practiced, combining traits from different lines in hybrids (Hartmann, 1988). The continu- ous development of inbred lines in poultry is expensive. The unpredictable outcome of ‘random’ crosses was the main weakness of the traditional use of line crosses and it was therefore rapidly replaced by other methods (Bell et al., Abstract. Reciprocal effects for sexual maturity, egg pro- duction, egg quality traits and viability are well known in poultry crosses. They have been used in an optimal way to form profitable production hybrids. These effects have been hypothesized to originate from sex-linked genes, maternal effects or a combination of both. However, these may not be the only explanations for reciprocal effects. Recent mapping of quantitative trait loci (QTL) has revealed autosomal areas with parent-of-origin specific effects in the chicken. In mammals, parental imprinting, i.e. the specifically regulat- ed expression of either maternal or paternal allele in the offspring, is the main cause of such effects. The most com- monly accepted hypothesis for the origin of imprinting, the conflict hypothesis, assumes a genetic conflict of interest between the maternal and paternal genomes regarding the allocation of resources to the offspring. It also intrinsically implies that imprinting should not occur in oviparous taxa. However, new molecular genetic information has raised a need to review the possible involvement of imprinting or Request reprints from Maria Tuiskula-Haavisto MTT Agrifood Research Finland, Biotechnology and Food Research H Building, FI–31600 Jokioinen (Finland) telephone: +358 3 4188 3623; fax: +358 3 4188 3618 e-mail: maria.tuiskula-haavisto@mtt.fi © 2007 S. Karger AG, Basel 1424–8581/07/1174–0305$23.50/0 Accessible online at: www.karger.com/cgr some related phenomena as a putative cause of reciprocal effects in poultry. Comparative mapping provides strong evidence for the conservation of orthologous imprinted gene clusters on chicken macrochromosomes. Further- more, these gene clusters exhibit asynchronous DNA repli- cation, an epigenetic mark specific for all imprinted regions. It has been proposed that these intrinsic chromosomal properties have been important for the evolution of im- printed gene expression in the mammalian lineage. Many of the mapped parent-of-origin specific QTL effects in chicken locate in or close to these conserved regions that show some of the basic features involved in monoallelic ex- pression. If monoallelic expression in these regions would be observed in birds, the actual mechanism and cause may be different from the imprinting that evolved later in the mammalian lineage. In this review we discuss recent mo- lecular genetic results that may provide tools for under- standing of reciprocal differences in poultry breeding and the evolution of imprinting. Copyright © 2007 S. Karger AG, Basel In this paper we review the use of reciprocal effects in poultry breeding, the basics of allele specific expression in- cluding imprinting, recent results from QTL mapping stud- ies suggesting parent-of-origin effects in birds, and the mo- Manuscript received 6 July 2006; accepted in revised form for publication by J. Smith, 6 October 2006.