Abstract
Gene-centromere (G-C) mapping provides insights into structural and behavioural properties of chromosomes. In this study, G-C mapping using microsatellite markers and meiogynogenetic (meiotic gynogenetic) families were performed in bighead carp (Aristichthys nobilis, 2N = 48), which belongs to Cyprinidae. A total of 218 microsatellites were selected across 24 linkage groups (LGs) of a recently well-defined genetic linkage map for bighead carp, with 151 being heterozygous in at least one of six dams in diploid meiogynogenetic families. After tests for Mendelian segregation in two diploid control families, 103 microsatellites were used for G-C distance calculation in 383 gynogens. The second division segregation frequency (y) was computed through half-tetrad analyses, and the values ranged from 0 to 0.97 (mean 0.40). High G-C recombination frequencies (over 0.667) were observed in 18 (17.5%) of the loci examined, which revealed a low level of chiasma interferences compared with other fishes studied previously. Distribution of G-C distances across LGs ranged from 0 cM to 48.5 cM (mean 20 cM) under the assumption of complete interference. All 24 centromeres were localized according to their closest-related microsatellites at 95% confident intervals. The average distance between centromeres and their closest-linked markers was 6.1 cM with 15 out of 24 LGs having a distance below 5 cM. Based on the centromere positions in this study, we proposed a formula of 24 m/sm+24 t/st chromosomes with 92 arms for bighead carp, which was mostly in accordance with a previously reported karyotype for bighead carp (24 m/sm+24 st). These results of centromere localization provide a basic framework and important resources for genetics and comparative genomics studies in bighead carp and its closely-related cyprinid species.
Highlights
Genetic mapping provides a framework for studies of quantitative trait loci (QTL) identification [1], comparative genome mapping [2], genome assembly and position-based cloning [3]
To obtain more reliable segregation data in meiogynogenetic families, those markers with possible null alleles were eliminated for recombination analysis, no matter they were in accordance with Mendelian expectations or not
Verification of Meiogynogenesis Common carp-specific alleles were observed in a total of 13 progenies from families A, B, E and F (Table 1), while no such alleles were detected in families C and D
Summary
Genetic mapping provides a framework for studies of quantitative trait loci (QTL) identification [1], comparative genome mapping [2], genome assembly and position-based cloning [3]. Genetic linkage maps only provide a reference to landmarks along the physical surface of a chromosome without a knowledge of centromere position [7]. The approach of half-tetrad analysis is the basis for G-C mapping, only if two of the four chromatids from a single meiosis were recovered, half-tetrad analysis could be performed [10]. By inhibiting release of the second polar body in fish and some other aquatic organisms, gynogenetic diploids or triploids can be produced and applied for the analysis of meiosis II (MII) half-tetrads [11]. With the nondisjunctions of the second polar body during MII, a dam heterozygous at a particular co-dominant marker locus should produce only two homozygous gametes when no crossovers occur between the marker and the centromere, but if crossovers occurred during meiosis I (MI), heterozygous gametes should emerge. For the G-C distance estimation, three mapping functions including complete interference [14], 50% interference [15] and no interference [16] can be applied
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