Nerve growth factor and Trk high affinity receptor (TrkA) gene expression in inflammatory bowel disease
Authors: F F di Mola
Journal: Gut (2000)

<h3>Abstract</h3> Repetitive DNA represents an important driver of sex chromosome differentiation. Yet, repetitive sequences tend to be misrepresented or overlooked in genomic studies. We analysed repetitive DNA landscape of sex chromosomes in several populations of a turquoise killifish <i>Nothobranchius furzeri</i> and its sister species <i>N. kadleci</i> (Teleostei: Nothobranchiidae), representatives of African annual killifishes with high rate of karyotype and sex chromosome evolution. We combined bioinformatic analyses of repeatome with molecular cytogenetic techniques such as comparative genomic hybridization, fluorescence <i>in situ</i> hybridization with satellite sequences, genes for ribosomal RNAs (rDNA) and bacterial artificial chromosomes (BACs) and immunostaining of SYCP3 and MLH1 proteins, which marked lateral elements of synaptonemal complexes and recombination sites, respectively. We revealed that <i>N. furzeri</i> and <i>N. kadleci</i> share the XY sex chromosome system, which is thus much older than previously assumed. Sex chromosomes are mostly heteromorphic as evidenced by distinct distribution of satellite DNAs and major rDNA. Yet, the heteromorphic XY sex chromosomes pair almost exclusively regularly in meiosis, which implies synaptic adjustment. Physical mapping of BACs identified inversions on Y chromosomes of the <i>N. kadleci</i> populations, akin to the pattern previously reported in <i>N. furzeri</i>. Yet, repetitive DNA landscape of X and Y sex chromosomes either diverged in parallel in populations of both species or it evolved in their common ancestor and thus predates the inversions. The observed differentiation via repeat repatterning thus cannot be explained by the classical sexually antagonistic model. Rather, we hypothesized that relaxed meiotic drive and recombination reduced by neutral processes could drive changes in repeatome and secondary inversions could be maintained by sexually antagonistic regulatory effects resulting from early evolution of dosage compensation. <h3>Author summary</h3> Early differentiation of sex chromosomes is not yet satisfactorily understood despite intensive research effort. Homomorphic sex chromosomes and their rapid turnover are common in teleost fishes, which makes them excellent models for studying evolution of nascent sex chromosomes. We investigated sex chromosomes in several populations of two sister species of African annual killifishes, <i>Nothobranchius furzeri</i> and <i>N. kadleci</i>, particularly their repetitive landscape, which was misrepresented in previous genomic studies. Combination of cytogenetic and genomic approaches revealed that both species share heteromorphic XY sex chromosome system. The <i>N. furzeri</i> XY sex chromosomes thus evolved earlier than previously expected. In <i>N. kadleci</i>, Y-linked inversions analogous to those reported in <i>N. furzeri</i> were detected. Changes in repetitive DNA distribution on sex chromosomes are either convergent or occurred in a common ancestor of both species, prior to the inversion events. The observed sex chromosome differentiation on repetitive DNA level thus cannot be reconciled with the classical theoretical model of sex chromosome evolution driven by sexually antagonistic selection. We invoke alternative explanations such as relaxed meiotic drive and recombination reduced by neutral processes, and we hypothesize that secondary inversions could be maintained by early evolution of dosage compensation resulting in sexually antagonistic regulatory effects.