Abstract
Sex determination in the Nile tilapia Oreochromis niloticus is more complex than a simple XX-XY sex determining mechanism, as evidenced from fairly frequent unexpected sex ratios in progeny. The production of uniform, homozygous experimental material is particularly advantageous for studying sex determining mechanism as well as for the genetic mapping and genome sequencing studies in which interpretations are facilitated by homozygosity. To better understand the genetic mechanism of sex determination, a fully inbred line of clonal females (XX) was verified in controlled environmental conditions using test crosses and microsatellite DNA markers from the tilapia linkage map. A total of successfully amplified 87 microsatellite DNA markers covering all 24 linkage groups were selected for screening sexually mature females from this line. 67 markers were found polymorphic in outbred individuals screened. Markers from LG1, LG3 and LG23 were given more emphasis because sex determining genes have been mapped on these LGs in different species of tilapia. The verification and validation of this clonal line of females made them an important resource to use as a standard reference line in genomics, sex determination studies and other studies in Nile tilapia. DOI: http://dx.doi.org/10.3329/ralf.v1i1.22378 Res. Agric., Livest. Fish.1(1): 147-158, Dec 2014
Highlights
Clonal individuals play a significant and expanding role in many scientific disciplines, including biomedicine, genomics, toxicology, immunology and evolutionary biology (Trevarrow and Robinson, 2004)
Fish species that have external fertilization can be reproduced by induced parthenogenesis, e.g., androgenesis or “mitotic” gynogenesis that may result in clonal individuals or novel genotypes
Clonal lines have been validated using a small number of markers but studies using larger numbers of polymorphic microsatellite markers to verify clonal lines of females are lacking
Summary
Clonal individuals play a significant and expanding role in many scientific disciplines, including biomedicine, genomics, toxicology, immunology and evolutionary biology (Trevarrow and Robinson, 2004). If the shock is given later, the first or the second mitotic division is suppressed (in mitotic gynogenesis or androgenesis) and two haploid copies of the maternal or paternal chromosomes respectively are retained to produce double haploid or dihaploid offspring These offspring carry only the duplicated set of chromosomes and are, by definition, fully homozygous individuals. The dihaploids should be homozygous at all loci but different individuals originating from the same outbred parent will be fixed for different alleles at any given locus depending on the recombination events that generated that gamete These dihaploid individuals from either a gynogenetic or androgenetic background can be used to generate clonal or isogenic lines as all gametes produced by such an individual will be identical, even after recombination (barring any mutation, which is expected to be very infrequent). It may be easier to hormonally sex reverse a proportion of the fish within a line to obtain both sexes (second generation or later), which allows the line to be propagated by simple crosses
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