Measurement of Chromosomal Arms and FISH Reveal Complex Genome Architecture and Standardized Karyotype of Model Fish, Genus Carassius.

Martin Knytl,Nicola Reinaldo Fornaini

doi:10.3390/cells10092343

Abstract

The widely distributed ray-finned fish genus Carassius is very well known due to its unique biological characteristics such as polyploidy, clonality, and/or interspecies hybridization. These biological characteristics have enabled Carassius species to be successfully widespread over relatively short period of evolutionary time. Therefore, this fish model deserves to be the center of attention in the research field. Some studies have already described the Carassius karyotype, but results are inconsistent in the number of morphological categories for individual chromosomes. We investigated three focal species: Carassius auratus, C. carassius and C. gibelio with the aim to describe their standardized diploid karyotypes, and to study their evolutionary relationships using cytogenetic tools. We measured length ( ) of each chromosome and calculated centromeric index (i value). We found: (i) The relationship between and i value showed higher similarity of C. auratus and C. carassius. (ii) The variability of i value within each chromosome expressed by means of the first quartile () up to the third quartile () showed higher similarity of C. carassius and C. gibelio. (iii) The fluorescent in situ hybridization (FISH) analysis revealed higher similarity of C. auratus and C. gibelio. (iv) Standardized karyotype formula described using value () showed differentiation among all investigated species: C. auratus had 24 metacentric (m), 40 submetacentric (), 2 subtelocentric (), 2 acrocentric (a) and 32 telocentric (T) chromosomes (); C. carassius: ; and C. gibelio: . (v) We developed R scripts applicable for the description of standardized karyotype for any other species. The diverse results indicated unprecedented complex genomic and chromosomal architecture in the genus Carassius probably influenced by its unique biological characteristics which make the study of evolutionary relationships more difficult than it has been originally postulated.

Highlights

Karyotype analysis is a fundamental approach by which chromosomes are arranged into homologous pairs with a respect to certain morphological categories
If the centromere is situated in median, submedian, subterminal or terminal region of the chromosome, the morphological category might be designated as metacentric (M, m), submetacentric, subtelocentric or acrocentric (a)/telocentric (t, T), respectively [2]
All steps outlining how the measured values were calculated and processed into tables and plots are shown on the C. auratus dataset

Summary

Introduction

Karyotype analysis is a fundamental approach by which chromosomes are arranged into homologous pairs with a respect to certain morphological categories. Homologous chromosome pairs and morphological categories are determined based on the ratio of the long (q) and short (p) arm and the position of the centromere (centromeric index, i value). If the centromere is situated in median, submedian, subterminal or terminal region of the chromosome, the morphological category might be designated as metacentric (M, m), submetacentric (sm), subtelocentric (st) or acrocentric (a)/telocentric (t, T), respectively [2]. Two edge points that are located on median and terminal position sensu stricto have centromeric index 50 (assigned as M) and 0 (assigned as T chromosomes), respectively [2]. The term karyogram can be used for a graphical depiction of chromosome complements [3,4]

Methods

Results

Discussion

Conclusion