Sharks, ancient and diverse inhabitants of our oceans, captivate with their unique biology. Like all organisms, these cartilaginous fishes possess a distinct genetic blueprint. This genetic information is organized within structures inside their cells. This article explores the number of these structures, called chromosomes, found in various shark species.
Understanding Chromosomes
Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA), which carries an organism’s genetic instructions. These structures organize DNA, ensuring efficient packaging and accurate transmission during cell division. Chromosomes carry genes, segments of DNA that determine specific traits. While the number of chromosomes is generally constant for a given species, it can differ significantly among different species.
The Range of Chromosome Numbers in Sharks
There is not a single, universal chromosome number for all shark species due to their wide diversity. The diploid number (2n) represents the total number of chromosomes in a somatic cell, consisting of two sets, one from each parent. Scientific studies, often using karyotyping, have revealed a broad spectrum of counts across different shark lineages.
For example, the great white shark has been reported to have 82 chromosomes (2n = 82), while the zebra shark has a diploid number of 102 (2n = 102). Some species within the Squaliformes order exhibit a range of haploid chromosome numbers (n) from 29 to 44, demonstrating considerable variation even within broader shark groups.
Further studies have estimated that the common ancestor of all extant sharks likely had a haploid chromosome number (n) of approximately 57. The oldest evolutionary lineages within both the Galeomorphii (which includes species like bull sharks and hammerheads) and Squalomorphii (which includes dogfish and sleeper sharks) clades maintained similar values, around 53–56 chromosomes, approximating this ancestral state.
Factors Influencing Chromosome Variation in Sharks
The observed variation in chromosome numbers among different shark species is influenced by various evolutionary processes. Chromosomal rearrangements, such as fusions, fissions, and inversions, can alter the number and structure of chromosomes over long periods. Chromosomal fusions occur when two chromosomes join to form a single, larger chromosome, while fissions involve a single chromosome splitting into two separate ones. These changes do not necessarily alter the total amount of genetic material but can significantly change the chromosome count.
Changes in chromosome number can also occur through aneuploidy, which involves the gain or loss of one or a few chromosomes, often due to errors during cell division like nondisjunction. The vast diversity of shark species has allowed these genetic changes to accumulate over millions of years. The primary mechanism of chromosome number change identified in sharks has been descending disploidy, which involves chromosome loss, particularly evident during the diversification of Carcharhiniformes and Squaliformes.