71 Annals of Applied Biology (2005), 146:71–79 *Corresponding Author E-mail: carputo@unina.it © 2005 Association of Applied Biologists Ploidy level manipulations in potato through sexual hybridisation D CARPUTO* and AMALIA BARONE Department of Soil, Plant and Environmental Sciences, University of Naples «Federico II», Via Università 100, 80055 Portici, Italy Summary There is no better use of sexual reproduction in regard to breeding and genetic research than the ploidy level manipulations possible in the potato and its relatives. Unique reproductive characteristics of tuber-bearing Solanum species make possible: the production of gametes with unreduced chromosome number; the presence of an endosperm dosage system that regulates success of interploidy/interspecific crosses; the possibility to easily extract maternal haploids following crosses with S. phureja. This paper reviews results obtained in scaling genomic multiples up and down in potato, and relates these manipulations to breeding strategies for the genetic improvement of the cultivated potato. Several ploidy series have been developed, ranging from the monoploid to the hexaploid level. Cultivated tetraploids were scaled down to the diploid and monoploid level by haploidy. Scaling upward was achieved by sexual polyploidisation via 2n gametes that resulted in triploid, tetraploid, pentaploid, and hexaploid genotypes with a broad genetic base. Altogether, the success of ploidy level manipulations constitutes further proof that sexual polyploidisation played an important role in the polyploid evolution of Solanum species, and supports the idea that gene flow can be relatively easily accomplished through interploid and bridge crosses. Key words: 2n gametes, endosperm, Solanum species, sexual polyploidisation, germplasm introgression Introduction Scaling up and down whole chromosome sets represents a powerful strategy to produce new genetic material for breeding purposes and genetic studies (Ramanna & Jacobsen, 2003). Among cultivated crops, the potato (Solanum tuberosum, 2n=4x=48) is one of the species where ploidy levels can be more easily manipulated and where this approach can have the widest multidisciplinary application. This is deemed very important given that the potato is unrivalled, among economic plants, in its abundance of related germplasm existing in nature at various ploidy levels (from diploid to hexaploid). Solanum species grow in a wide range of environments, from the southern part of the United States to southern Chile, and possess many desirable traits for potato breeding. The possibility of manipulating chromosome sets in the potato is essentially based on three biological features. First of all, there are several genotypes (mainly of Series Tuberosa) that produce gametes with an unreduced chromosome number (2n gametes) as the result of meiotic anomalies affecting either micro- or macrosporogenesis. The genetic control of these meiotic mutations is still an open question. According to some authors (reviewed in Peloquin et al., 1999), they are controlled by recessive genes with incomplete penetrance and variable expressivity. The presence of two genetic systems has also been hypothesised (Carputo et al., 1995). Analysis of alpha-tubulin and F-actin distribution during cytokinesis of meiotic mutants provided evidence that, besides spindle orientation, microtubular cytoskeleton abnormalities are involved in the formation of 2n pollen (Genualdo et al., 1998). Other scientists hypothesised that the production of 2n gametes behaves as a quantitative trait (Ramanna, 1983). What is important in this context is that 2n gametes are the basis for sexual polyploidisation events. Bilateral and unilateral sexual polyploidisation (BSP and USP, respectively) can occur spontaneously in nature or can be induced artificially through controlled pollinations. BSP involves crosses where the female parent forms 2n eggs and the male one 2n pollen. By contrast, USP is based on crosses where only one parent produces 2n gametes (either 2n pollen or 2n eggs). The second important biological characteristic of the potato is the endosperm balance number (EBN) (Johnston et al., 1980) that influences the success of interploidy/interspecific crosses. The EBN is a number varying from 1 to 4, and represents the “effective ploidy” of Solanum species. Cultivated S. tuberosum is 4EBN, whereas most wild species (either diploid or tetraploid) are 2EBN. According to the model developed, interploidy/interspecific crosses give a normal endosperm development only when there is a 2 : 1 maternal to paternal EBN ratio