Proceedings of the XLV Italian Society of Agricultural Genetics - SIGA Annual Congress

Salsomaggiore Terme, Italy - 26/29 September, 2001

ISBN 88-900622-1-5

Poster Abstract

GENES AND GENETIC ANALYSES OF IMPRINTABLE ZEIN SEQUENCES (IZS) IN MAIZE ENDOSPERM

CASTELLI S., BAGGIO E., OCCHIPINTI E., COSENTINO C., LAZZARI B., VIOTTI A.

Istituto Biosintesi Vegetali, CNR, Via Bassini 15, 20133 Milano, Italy

castelli@ibv.mi.cnr.it; lazzari@ibv.mi.cnr.it; cosentino@ibv.mi.cnr.it; aviotti@ibv.mi.cnr.it

uniparental expression, endosperm, zein genes

Molecular and genetic evidences indicate that in animals and plants several genetic traits are expressed uniparentally. The parent of origin dependent gene expression frequently derives from traits distributed unequally to the progenies or less frequently to gametic imprinting, an epigenetic phenomenon that specifically marks the parent alleles. In plants, most of the evidences involve genes expressed in the endosperm and in particular for those traits that specify colouring and storage products. Uniparental expression can be mediated either by post-transcriptional gene silencing (PTGS) or transcriptional gene silencing (TGS). Maize genotypes accumulate different zein polypeptides in the endosperm. Each line shows a specific two-dimensional zein pattern upon fractionation by IEF/SDS-PAGE. This analysis resolves the heavy (zH) and the light (zL) chain zein classes into several polypeptides (10 to 15 spots for each class) of different pI. Analyses of reciprocal crosses between several lines show that certain zH spots of some genotypes are absent when contributed as paternal complement to specific maternal genotypes, whereas they always occur in the self or when contributed as maternal complements independently from the paternal lines. This unidirectional cross-specific silencing identifies: i) an imprinted cross(es) determined by an imprinting line(s) that expresses a repressing maternal factor(s) and ii) Imprintable zein sequences (Izs) genotype-specific. PCR and RT-PCR analyses confirm the presence of the silenced sequences in the genomes of the reciprocals and the absence of their transcripts in the imprinted cross, suggesting that the silencing occurs by a TGS mechanism. Clones of zein transcripts from parent lines and non-imprinted crosses were established in a vector that allows unidirectional insertion of zein sequences and their transcription for proper translation and molecular analyses. The clones were characterised by their insert length, restriction sites, in vitro/in vivo translation products, coding capacities and computer-deduced versus experimental pIs. Specific primers for each Izs were designed and used to analyse their modification state in the 1n, 2n and 3n genomes of the parent lines and of their reciprocals. Genetic analyses of Izs of both F1s (imprinted cross and non-imprinted cross) indicate that Izs maintain their imprintable state in the respect of the imprinting line (silenced when used as paternal complement) while the imprinting line loses the imprinting factors when derives from hybrids. The silencing of Izs occurs always on paternal derived alleles supporting the parental conflict theory reported by Haigh and Westoby suggesting, however, a more specific behaviour based on genotypes.