How Many Chromosomes Do Sharks Have? Shark Karyotypes Explained

Sharks, among the oldest vertebrates, display remarkable genetic diversity. Their chromosome numbers vary widely across species, offering valuable insights into their evolution, reproduction, and conservation. Studying their karyotypes helps researchers understand genome organization and adaptation.

Mapping shark genomes presents challenges due to difficulties in obtaining high-quality samples. However, advancements in cytogenetic techniques have allowed scientists to compare chromosomal structures across different shark families.

Range Of Chromosome Numbers In Sharks

Sharks exhibit significant variation in chromosome numbers, reflecting their evolutionary history and ecological adaptations. Diploid counts range from 30 to over 100, even among closely related species. For example, the shortfin mako (Isurus oxyrinchus) has 82 chromosomes, while the whale shark (Rhincodon typus) has 88. These differences suggest that chromosomal rearrangements, such as fusions and fissions, have shaped shark genomes.

Chromosome numbers often align with taxonomic classification. Members of the family Carcharhinidae, including the tiger shark (Galeocerdo cuvier) and bull shark (Carcharhinus leucas), typically have diploid counts between 70 and 80. In contrast, Lamnidae species, such as the great white shark (Carcharodon carcharias), generally possess higher counts. These variations may stem from evolutionary pressures related to habitat, reproduction, and metabolism.

Polyploidy, or possessing multiple chromosome sets, is rare in sharks but has been hypothesized in some deep-sea species. Some researchers propose that certain groups may have undergone chromosomal duplications, enhancing their resilience in extreme environments. While direct evidence remains limited, structural variations like inversions and translocations indicate ongoing chromosomal evolution. Molecular cytogenetics has shown that repetitive DNA sequences and centromeric regions help maintain genome stability despite these changes.

Karyotyping Methods In Shark Studies

Analyzing shark chromosomes is challenging due to their cartilaginous skeletons, which complicate traditional cytogenetic techniques. Researchers use specialized methods to obtain mitotic cells from fin clips, gill tissue, or blood samples. These cells are cultured under optimized conditions to induce division, a necessary step for karyotyping.

Once dividing cells are available, chromosome preparations require precise handling. Hypotonic treatments, using potassium chloride or sodium citrate, swell cells for clearer observation. Fixation in methanol-acetic acid preserves chromosomal integrity, while air-drying enhances spreading. Giemsa banding (G-banding) helps distinguish individual chromosomes and identify structural rearrangements such as translocations, deletions, and inversions.

Fluorescence in situ hybridization (FISH) has improved shark karyotyping by allowing researchers to map specific DNA sequences. This technique uses fluorescent probes to highlight target regions, aiding in the study of repetitive elements, centromeres, and gene loci. Comparative genomic hybridization (CGH) has also been used to detect copy number variations. These molecular tools reveal that sharks generally have stable karyotypes compared to many teleost fish, though some lineages show notable chromosomal rearrangements.

Comparisons Among Different Shark Families

Chromosome numbers and structures vary across shark families, reflecting their evolutionary paths and ecological specializations. Among well-studied groups, Carcharhinidae and Lamnidae show distinct karyotypic differences. Carcharhinid sharks, such as the blacktip shark (Carcharhinus limbatus), typically have diploid counts between 70 and 80, suggesting relative chromosomal stability over time. This consistency may be due to the similar ecological conditions these species inhabit.

Lamnids, including the great white shark (Carcharodon carcharias), often have higher chromosome numbers, exceeding 80. This difference could result from past chromosomal fissions or duplications, potentially linked to adaptations like regional endothermy. Unlike carcharhinids, lamnids regulate their body temperature, which may be associated with genomic modifications affecting metabolism and muscle function. While no direct link between chromosome number and thermoregulation has been established, these karyotypic patterns indicate deeper genomic distinctions.

Other families further illustrate chromosomal diversity. Squalidae (dogfish sharks), such as the spiny dogfish (Squalus acanthias), tend to have lower chromosome counts, sometimes below 50, likely due to chromosomal fusions. In contrast, Hexanchidae (six- and seven-gill sharks) possess some of the highest chromosome numbers in sharks, with diploid counts exceeding 100 in certain species. Their complex karyotypes suggest an ancestral genome structure that has remained largely unchanged, preserving ancient genetic traits that support survival in deep-sea environments.