A CONSENSUS MAP FOR CUCURBITA PEPO L.- THE GENETICS OF THE SEED COAT IN C. PEPO

A genetic map for Cucurbita pepo L. (2n=40) is a useful tool for immediate application in plant breeding and comparative genetic analysis. Using Random Amplified Polymorphic DNA (RAPD), Amplified Fragment Length Polymorphism (AFLP), Simple Sequence Repeats (SSR) and morphological traits, two DNA marker maps for Cucurbita pepo were constructed. In total 333 and 323 markers were mapped in zucchini and crookneck maps, respectively. The remaining markers are for the time being still unlinked or grouped in doublets and triplets. The two populations shared 92 loci, 62 of which were mapped. These loci enabled the comparison of the two maps and established homology between 14 linkage groups. The order of these loci was in most cases similar, but inversions and rearrangements were observed. Some of these markers could play the role of anchor loci for Cucurbita pepo. Bulked Segregant Analysis (BSA) identified seven markers linked to the seed coat character (less than 7 cM) in the zucchini population. One of these markers was found linked to the seed coat (less than 4 cM) in the crookneck population. The seed coat character mapped to a linkage group that contains 4 shared markers, suggesting the usefulness of common markers to play the role of anchor loci. Two (Sequence Characterized Amplified Region) SCAR markers linked to Zucchini Yellow Mosaic Virus (ZYMV) resistance genes were mapped in the zucchini map in two different linkage groups together with a number of clustered loci originating from the source of resistance Cucurbita moschata. The genetics of the seed coat character was studied on progenies derived from three crosses between oil-pumpkin varieties as the hull-less seeded parents and two zucchini and one crookneck varieties as the hulled seeded parents. All F1s were selfed to produce F2populations. In one case F3-, F4and F5-progenies were produced. The seed coat of the F1s was of a completely hulled type and segregated in the next generation 3:1 hulled vs. hullless. However, the seed coat phenotype of some of the hull-less F2-plants was not completely hull-less exhibiting a residual lignification. Following a visual examination of the hull-less progeny, seed coat types were classified in 3 categories, the segregation of which did not fit the 1:2:1 segregation of an incomplete dominant minor gene. Segregation of the hull-less categories in F3, F4 and F5 showed a seemingly random segregation of the three categories disregarding the category from which they have been derived. Four hull-less genotypes from different geographic origins were crossed in six combinations. Their F2-progenies did not show any segregation with respect to the seed coat character and almost all F2-plants seeds were of a completely hull-less type. This finding supports the common origin of the hull-less type that emerged most probably in south-east Austria. Histological investigations confirmed the visual categorizing, and revealed the effect of the major dominant gene at the histological level of the seed testa. The residual lignification observed on the seeds of some of the hull-less segregants was exclusively due to the presence of a single lignified sclerenchyma cell layer which appears also as a second layer in the hulled segregants. The amount of lignin deposited on this cell layer seems to be a QTL showing a continuous variation on both hull-less and hulled seeds. It did not play any role in the differentiation between the hulled and hull-less phenotypes since the most lignified sclerenchyma observed on some of the hull-less seeds showed similar intensity as compared to the least lignified hulled seeds. The process of lignification might be regulated at the transcriptional level as evidenced by the presence of seeds with varying phenotypes in the fruit(s) of the same individual plant.