Post-zygotic gametic selection due to endosperm balance number explains unusual chromosome numbers of 3� � 2� progeny in Solanum

Abstract

Crosses between triploid and diploid genotypes are usually the best sources of trisomics in potato as well as in several other crop species. However, 3× × 2× crosses between triploid (2n=3×=36; 2EBN) Solanum commersonii-S. tuberosum hybrids and diploid (2n= 2×=24; 2EBN) genotypes gave progenies with a high number of extra chromosomes, 29-36, suggesting that only eggs with 17-24 chromosomes produced embryos that reached full development. Our hypothesis is that although triploids produce eggs with a range of chromosome numbers, 3× × 2× crosses involving a 2×(2EBN) parent favor eggs with a high chromosome number. These eggs have higher probabilities of possessing the same endosperm balance number (EBN) value (i.e. 1) of gametes produced by the 2EBN diploid parent to give the required 2:1 maternal to paternal EBN ratio in the hybrid endosperm. Under this model, trisomics are produced only if the diploid parent has an EBN of 1. Based on our results and those reported in the literature, it is proposed that in 3×(2EBN) × 2×(2EBN) crosses the endosperm balance number exercises negative selection for gametes with a low chromosome number, and a corresponding low EBN, and positive selection for gametes with a high chromosome number and EBN.& k w d : Key words Endosperm breakdown · Aneuploidy · 3× × 2× crosses · Solanum commersonii · Solanum tuberosum& b d y :

Pairing analysis and in situ Hybridisation reveal autopolyploid-like behaviour in Solanum commersonii × S. tuberosum (potato) interspecific hybrids

“…It is not clear yet what factors determine EBN or where they are mapped in the potato genome. Aneuploid gametes with extra chromosomes bearing the EBN controlling factors might be positively selected for compatible crosses and thus favour the formation of genotypes with complements higher than the expected 2n = 5x = 60 (Carputo 1999;Henry et al 2009). For the BC 2 plants Carputo et al (2003) observed a narrower range of chromosome number variation, with a tendency towards tetraploid (2n = 4x = 48) numbers.…”

Ploidy level manipulations in potato through sexual hybridisation

“…The hybrids developed have 50% wild genome, and can be used in USP or BSP crossing schemes to re-establish the chromosome number of the cultivated potato, and gradually reduce the wild genome content. In the case of 3x × 2x crosses between triploids with EBN=2 and diploids with same EBN, progenies with a high number of extra chromosomes were obtained (Carputo, 1999). It should be pointed out that haploid-wild species hybrids represent a tool to indirectly evaluate tuber characteristics of wild species that lack tuberisation under long days (Yerk & Peloquin, 1989), as well as phenotypic correlations among traits of interest (Serquen & Peloquin, 1996).…”

The reproductive biology of the potato and its implication for breeding

“…In this model, genomes have been assigned values of 1/2 or 1 (Ehlenfeldt & Hanneman, 1984); therefore, 2x(1EBN) species are described as 1/2-1/2, 2x(2EBN) as 1ol, 4x(2EBN) as 1/2ol/2ol/2ol/2, 4x(4EBN) as 1~176176 and 6x(4EBN) as 1/2~176176 Bamberg & Hanneman (1990) were unable to induce recombination or segregation for EBN in artificial hybrids of North and South American species, and concluded that the genomes of both groups carry EBN in a genetically similar way. EBN has also been noted as being involved in post gametic selection favoring those matching the EBN of the male parent in inter-EBN crosses (Carputo, 1999). Camadro & Masuelli (1995) have also proposed a genetic model for'EBN to explain their interspecific crossing results with 4x S. acaule, 2x S. commersonii, 2x S. gourlayi and 4x S. gourlayi.…”

“…This is because mutations leading to a new EBN would either be lethal due to endosperm failure or would form a new true-breeding, genetically isolated but sympatric population (if fertilized by gametes having the new EBN). EBN has been recognized as a strong isolating mechanism in sexual reproduction (Carputo, 1999). 2n gametes.…”

“…They noted that EBN could serve as both a species initiator and a maintainer of species identity. Carputo (1999) has tied this together with an emphasis on the role of 2n gametes.…”