Integration of Genetic and Cytogenetic Maps and Identification of Sex Chromosome in Garden Asparagus (Asparagus officinalis L.).

Roberto Moreno,Marie Kubaláková,Petr Cápal,Jaroslav Doležel,Patricia Castro,Jan Vrána,Teresa Millán,Juan Gil,Verónica García

doi:10.3389/fpls.2018.01068

Abstract

A genetic linkage map of dioecious garden asparagus (Asparagus officinalis L., 2n = 2x = 20) was constructed using F1 population, simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers. In total, 1376 SNPs and 27 SSRs were used for genetic mapping. Two resulting parental maps contained 907 and 678 markers spanning 1947 and 1814 cM, for female and male parent, respectively, over ten linkage groups representing ten haploid chromosomes of the species. With the aim to anchor the ten genetic linkage groups to individual chromosomes and develop a tool to facilitate genome analysis and gene cloning, we have optimized a protocol for flow cytometric chromosome analysis and sorting in asparagus. The analysis of DAPI-stained suspensions of intact mitotic chromosomes by flow cytometry resulted in histograms of relative fluorescence intensity (flow karyotypes) comprising eight major peaks. The analysis of chromosome morphology and localization of 5S and 45S rDNA by FISH on flow-sorted chromosomes, revealed that four chromosomes (IV, V, VI, VIII) could be discriminated and sorted. Seventy-two SSR markers were used to characterize chromosome content of individual peaks on the flow karyotype. Out of them, 27 were included in the genetic linkage map and anchored genetic linkage groups to chromosomes. The sex determining locus was located on LG5, which was associated with peak V representing a chromosome with 5S rDNA locus. The results obtained in this study will support asparagus improvement by facilitating targeted marker development and gene isolation using flow-sorted chromosomes.

Highlights

Garden asparagus (Asparagus officinalis L., 2n = 2x = 20) is the most economically important species of the Asparagus genus with a cultivation area similar to other vegetable crops such as garlic, carrot and eggplant (FAOSTAT, 2016)
Sequencing the tGBS library constructed from the mapping population PS010 × WN124 generated 624.4 million reads
After trimming low quality bases, around 90% of sequence reads were used for single nucleotide polymorphism (SNP) discovery

Summary

Introduction

Garden asparagus (Asparagus officinalis L., 2n = 2x = 20) is the most economically important species of the Asparagus genus with a cultivation area similar to other vegetable crops such as garlic, carrot and eggplant (FAOSTAT, 2016). Garden asparagus is a dioecious perennial species and a single gene (M/m) is responsible for sex determination. Females are homozygous recessive (mm) whereas males are heterozygous (Mm) (Rick and Hanna, 1943) and the sex ratio in traditional clonal hybrids (mm: Mm) is 1:1. Sex is an important agronomical trait in this species because male plants are economically more profitable than female plants (Sneep, 1953; Benson, 1982; Ellison, 1986). Supermales can be obtained after selfing andromonoecious plants, i.e., male plants that bear normal staminate and bisexual flowers. Andromonoecious plants occur spontaneously at low frequency. The formation of androgenetic embryos is genotype dependent and the frequency of double haploid plants is generally low (Falavigna and Casali, 2002; Riccardi et al, 2011)

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