Salmonidae

Evolution

Current salmonids comprise three main clades taxonomically treated as subfamilies: Coregoninae (freshwater whitefishes), Thymallinae (graylings), and Salmoninae (trout, salmon, char, taimens and lenoks). Generally, all three lineages are accepted to allocate a suite of derived traits indicating a monophyletic group.

The suborder Salmonoidei, containing only Salmonidae, is one of two extant clades within the order Salmoniformes, which first appeared during the Santonian and Campanian stages of the Late Cretaceous. The other is the Esocoidei, which contains the pikes and mudminnows. Formerly, many more families were included within this group, but all have since been reclassified into their own orders. During the early 21st century, Salmoniformes was redefined to include only Salmonidae as a monotypic order. However, as recent phylogenetic studies have affirmed the relationship between the Salmonoidei and Esocoidei, the latter have returned to being considered a suborder of Salmoniformes.[[File:Eosalmo driftwoodensis 02.jpg|thumb|right|[[Eosalmo driftwoodensis]] fossil [[Klondike Mountain Formation]]]]It is thought that salmon and pike diverged from one another during the Cretaceous, but until 2025, there was no fossil evidence of salmonids occurring during this time period. In 2025, the earliest known fossil salmonid, Sivulliusalmo alaskensis, was described from the early Maastrichtian-aged Prince Creek Formation of Alaska, with indeterminate remains of this genus also being identified from the older Campanian-aged Dinosaur Park Formation of Alberta. The occurrence of Sivulliusalmo in these northern habitats suggests that the modern salmonid preference for cool, high-latitude waters is an ancient, conserved trait.

Prior to the description of Sivulliusalmo, the earliest record of salmonids was the Early Eocene-aged Eosalmo driftwoodensis, a stem-salmonine, which was first described from fossils found at Driftwood Creek, central British Columbia, and has been recovered from most sites in the Eocene Okanagan Highlands. This genus shares traits found in all three subfamily lineages. Hence, E. driftwoodensis is an archaic salmonid, representing an important stage in salmonid evolution. Fossil scales of coregonines are known from the Late Eocene or Early Oligocene of California.

A gap appears in the salmonine fossil record after E. driftwoodensis until about 7 million years ago (mya), in the Late Miocene, when trout-like fossils appear in Idaho, in the Clarkia Lake beds. Several of these species appear to be Oncorhynchus — the current genus for Pacific salmon and Pacific trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5–6 mya), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon and European trout) must have occurred well before the Pliocene. Suggestions have gone back as far as the Early Miocene (about 20 mya).

Genetics

Based on the most current evidence, salmonids diverged from the rest of teleost fish no later than 88 million years ago, during the late Cretaceous. This divergence was marked by a whole-genome duplication event in the ancestral salmonid, where the diploid ancestor became tetraploid. This duplication is the fourth of its kind to happen in the evolutionary lineage of the salmonids, with two having occurred commonly to all bony vertebrates, and another specifically in the teleost fishes.

Extant salmonids all show evidence of partial tetraploidy, as studies show the genome has undergone selection to regain a diploid state. Work done in the rainbow trout (Onchorhynchus mykiss) has shown that the genome is still partially-tetraploid. Around half of the duplicated protein-coding genes have been deleted, but all apparent miRNA sequences still show full duplication, with potential to influence regulation of the rainbow trout's genome. This pattern of partial tetraploidy is thought to be reflected in the rest of extant salmonids.

The earliest presumed crown-group salmonid fish (E. driftwoodensis) does not appear until the middle Eocene. This fossil already displays traits associated with extant salmonids, but as the genome of E. driftwoodensis cannot be sequenced, it cannot be confirmed if polyploidy was present in this animal at this point in time.

Given a lack of earlier transition fossils, and the inability to extract genomic data from specimens other than extant species, the dating of the whole-genome duplication event in salmonids was historically a very broad categorization of times, ranging from 25 to 100 million years in age. New advances in calibrated relaxed molecular clock analyses have allowed for a closer examination of the salmonid genome, and has allowed for a more precise dating of the whole-genome duplication of the group, that places the latest possible date for the event at 88 million years ago.

This more precise dating and examination of the salmonid whole-genome duplication event has allowed more speculation on the radiation of species within the group. Historically, the whole-genome duplication event was thought to be the reason for the variation within Salmonidae. Current evidence done with molecular clock analyses revealed that much of the speciation of the group occurred during periods of intense climate change associated with the last ice ages, with especially high speciation rates being observed in salmonids that developed an anadromous lifestyle.

Classification

Together with the closely related orders Esociformes (pikes and mudminnows), Osmeriformes (true smelts) and Argentiniformes (marine smelts and barreleyes), Salmoniformes comprise the superorder Protacanthopterygii.

The only extant family within Salmoniformes, Salmonidae, is divided into three subfamilies and around 10 genera containing about 220 species. The concepts of the number of species recognised vary among researchers and authorities; the numbers presented below represent the higher estimates of diversity:

|{{clade|style=font-size:90%;line-height:80%;width:400px |}}

Order Salmoniformes

• Family: Salmonidae
  • †Sivulliusalmo Brinkman et al., 2025 (1 species, Late Cretaceous)
  • Subfamily: Coregoninae
    • Coregonus Linnaeus, 1758 - whitefishes (78 species)
    • Prosopium Jordan, 1878 - round whitefishes (6 species)
    • Stenodus Richardson, 1836 - beloribitsa and nelma (2 species)
    • †Beckius David, 1946 (1 species, Oligocene)
    • †Parastenodus David, 1946 (1 species, Eocene)
  • Subfamily: Thymallinae
    • Thymallus Linck, 1790 - graylings (14 species)
  • Subfamily: Salmoninae
    • †Eosalmo Wilson, 1977 (1 species, Eocene)
    • †Paleolox Kimmel, 1975 (1 species, Late Miocene)
    • Tribe: Salmonini
      • Salmo Linnaeus, 1758 - Atlantic salmon and trout (47 species)
      • Salvelinus Richardson, 1836 - Char and trout (e.g. brook trout, lake trout) (51 species)
      • Salvethymus Chereshnev & Skopets, 1990 - Long-finned char (1 species)
    • Tribe: Oncorhynchini
      • Brachymystax Günther, 1866 - lenoks (4 species)
      • Hucho Günther, 1866 - taimens (4 species)
      • Oncorhynchus Suckley, 1861 - Pacific salmon and trout (12 species)
      • Parahucho Vladykov, 1963 - Sakhalin taimen (1 species)

Hybrid crossbreeding

The following table shows results of hybrid crossbreeding combination in Salmonidae.

note :- : The identical kind, O : (survivability), X : (Fatality)

References

References

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