A kiwifruit (Actinidia spp.) linkage map based on microsatellites and integrated with AFLP markers

A genetic map of kiwifruit (Actinidia spp.) was constructed using microsatellite and AFLP markers and the pseudo-testcross mapping strategy. (AC)n and (AG)n microsatellite repeats were first isolated from Actinidia chinensis (2n = 2x = 58) enriched genomic libraries and tested for segregation in the interspecific cross between the diploid distantly related species A. chinensis and A. callosa. Some 105 microsatellite loci of the 251 initially tested segregated in the progeny in a 1:1 ratio as in a classical backcross, or in a ratio which could mimic the backcross, and were mapped using 94 individuals. AFLP markers were then produced using MseI and EcoRI restriction enzymes and 15 primer combinations. Nearly 10% of loci showed a distorted segregation at α = 0.05, and only 4% at α = 0.01, irrespectively to the marker class. Two linkage maps were produced, one for each parent. The female map had 203 loci, of which 160 (71 SSR and 89 AFLP) constituted the framework map at a LOD score ≥ 2.0. The map was 1,758.5 cM(K) long, covering 46% of the estimated genome length. The male map had only 143 loci, of which 116 (28 SSR, 87 AFLP and the sex determinant) constituted the framework map. The map length was only 1,104.1 cM(K), covering 34% of the estimate genome length. Only 35 SSR loci were mapped in the male parent because 18% of SSR loci that were characterised did not amplify in A. callosa, and 48% were homozygous. The choice of parents in the pseudo-testcross is critically discussed. The sex determinant was mapped in A. callosa.