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

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Poster Abstract - 1.33

UNDERSTANDING THE GENOME CONSTITUTION OF TETRAPLOID SALIX BY GENETIC LINKAGE MAPPING AND GENOMIC IN SITU HYBRIDIZATION

MENEGHETTI S.*, BARCACCIA G.*, DEJONG H.**, LUCCHIN M.*

*) Dipartimento di Agronomia Ambientale e Produzioni Vegetali, Università di Padova, Agripolis, Via Romea 16, 35020 Legnaro, Padova, Italy

margherita.lucchin@unipd.it

**) Laboratory of Genetics, University of Wageningen, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands

willow, AFLP, GISH, polyploidy

The Salix alba (white willow) - Salix fragilis (crack willow) complex includes closely related polyploid species which are dioecious and thus obligate outcrossers. Natural populations of willow are represented by a mixture of highly heterozygous genotypes sharing a common gene pool. Because nothing is known about their genomic constitution, the tetraploid level (2n=4x=76) of willow species makes difficult basic and applied genetic studies. We have recently used a two-way pseudo-testcross strategy and single-dose markers to construct the first linkage maps for both male and female willows. In S. alba, 73 maternal and 48 paternal marker alleles were mapped to 19 and 16 linkage groups covering 708 cM and 339 cM, respectively. In S. fragilis, 13 maternal and 33 paternal marker alleles were mapped in 6 and 14 linkage groups covering 98 cM and 321 cM, respectively. For most cosegregation groups, a comparable number of markers linked in coupling and repulsion was identified. This finding suggests that most basic chromosomes pair preferentially as it happens for allotetraploid species with disomic inheritance. The detection of 10 couples of marker alleles of single-parents with co-dominant inheritance patterns strengthens this hypothesis. The fact that of the 1,122 marker loci totally identified in the two male and female parents, the vast majority (77.5%) were proven to be polymorphic and as few as 22.5% shared between parental species, highlights S. alba and S. fragilis genotypes deeply differentiated. The highly significant difference between S. alba-specific and S. fragilis-specific markers found in both parental combinations (on average, 65.3% vs. 34.7%, respectively) supports the phylogenetic hypothesis according which S. fragilis can be considered as a derivative from S. alba-like progenitors. Although the analysis of pairing behavior of tetraploid Salix spp. could help to explain the genomic constitution of willow, cytological observations in this species are difficult to perform owing to the high chromosome number and small chromosome size. Preliminary cytological observations carried out on chromosomes isolated from root tip meristems proved that about half of the chromosome complements at prometaphase are less contracted than the remaining chromosomes, which are already at almost complete contraction. Likely this reflects that chromosomes are from different homoelogous genomes and have different contraction ratios, supporting so that tetraploid S. alba and S. fragilis are allopolylploids. Genomic in situ hybridisations on chromosome sets of spread root tip and anther preparations are in progress to elucidate the genomic structure of pure willow species and the origin of parental species in the hybrids.