Chromosome evolution in the Salmonidae (Pisces): an update

Area of Science:

• * Ichthyology and Cytogenetics: Focus on the chromosomal characteristics of salmonid fishes.
• * Evolutionary Biology: Investigating the mechanisms of karyotype evolution in fish.

Background:

• * Salmonid fishes encompass three subfamilies: Coregoninae, Thymallinae, and Salmoninae.
• * Previous reviews (e.g., Hartley, 1987) established two primary karyotype categories (Type A and Type B) based on diploid chromosome numbers (2n) and chromosome arm numbers (NF).

Purpose of the Study:

• * To update the review of salmonid fish karyotypes.
• * To propose new subcategories for karyotypes based on variations in chromosome arm numbers.
• * To elucidate the distinct modes of chromosome evolution within salmonid subfamilies.

Main Methods:

• * Review and synthesis of existing karyotype data for salmonid fishes.
• * Classification of karyotypes into established and newly proposed categories (A, B, A', A", B').

Main Results:

• * Established Type A (2n ≈ 80, NF ≈ 100) and Type B (2n ≈ 60, NF ≈ 100) karyotypes are confirmed.
• * Proposed new subtypes: A' (NF = 110-120), A" (NF > 140), and B' (NF < 80).
• * Identified two primary evolutionary modes: Robertsonian centric fusions (Coregoninae, Salmoninae) and inversions (Thymallinae).

Conclusions:

• * Coregoninae and Salmoninae predominantly evolved via centric fusions, leading to Type A and B karyotypes.
• * Thymallinae exhibit karyotype evolution through inversions, resulting in Type A' karyotypes.
• * Type B karyotypes are common in the genus *Oncorhynchus* and associated with anadromous or specialized lacustrine life histories, potentially linked to selection on genetic recombination.