Prelimenary Study On The Origin Of The Sex Chromosome System In Collichthys Lucidus

Collichthys lucidusis one of the most important marine-cultured fish in southern part of China.Recently,the study of C.lucidus focused on the morphological difference,phylogeny and population genetics.However,the study about cytological of chromosomeremain unclear.We determined the karyotype by fluorescence staining.FISH mapping was carried out by using different kinds of hybridization probes in the C.luciduschromosome to find the sex chromosome label.The signal distribution could help us to analysis the sex chromosome mechanism.The results were as shown:1.DAPI dyeing and Self-Gish method were used to analyze the karyotype of the C.lucidus.The karyotype of female was 2n=48t,NF=48whereas the male was 2n=46t+1sm,NF=48.The analysis of Self-GISH demonstrates that the distribution of signal in 17 chromosome was the same as the 1 chromosome and Y chromosome,and the 17 chromosome was a monosomein male karyotype..2.By using the multicolor FISH method,we located the 18S rDNA、5S rDNA and telomeric sequences in the chromosomes of C.lucidus.And there ubiety were definitely clear.In the female,the signal distribution of 18S and 5S rDNA were in X₁ chromosome and there was a strong interstitialtelomere signal between two gene cluster.In the contrast,the signal distribution of 18S and 5S rDNA were in X₁chromatid and Y chromosome respectively in the male.And the interstitialtelomere signal could only be found in X₁chromatid.3.The FISH was performed to analysis the repetitive sequence（CA）₁₅、（CAA）₁₀、（CAG）n and（CAT）n.The results indicate that the signal of（CA）₁₅5 and（CAA）₁₀0 could be observed in all the terminal of the chromosome.There was a strong signal of the（CAT）n near the terminalof the X₁ chromosome.The fluorescence signal of（CAG）n could only be observed in X₁ chromosome and Y chromosome.4.Hybridization（Zoo-FISH）was carried out in the interphase nucleus of C.lucidusthe onbasis of large yellow croaker BAC-clone49L4 and 20F6.We found that the signal of20F6BAC-clone could not be detected,and observed weak signalof49L4BAC-clone in the interphase nucleus of C.Lucidus.These results demonstrated that the type of sex chromosome system of C.luciduswas X₁X₁X₂X₂/X₁X₂Y.The biggest telocentric centromere chromosomesis X₁ chromosome and there is amiddle centromere chromosome in male which is the Y chromosome.17 chromosome might as well be the X₂chromosome of C.lucidus.The difference of the signal distribution between the male and female indicates that the chromosome system of C.lucidus was formed by the deletion and fusion of the original chromosome.In addition,different types of repetitive sequences accumulate on sex chromosomes.