How Much Chromosomes Do Fishes Have? [Solved!]


Sharing is Caring


How much do fish chromosomes vary? Do they have 2n or 2n + 2n? These are some of the most burning questions in fish genetics. While you may know the general answer (that fish chromosomes count for 2 and are thus diploid), you might be surprised by the more detailed implications of this simple fact. For example, diploidy is frequently associated with genetic monogamy. This fact will surprise you, as you may have thought that genetic monogamy was more common in species having multiple sex partners. While this may be true for promiscuous fishes (e.g., the blue swimmer), it is certainly not the case for the vast majority of fishes. For example, among freshwater fishes, there is only one other case of genetic monogamy (bony fishes–fugu, pike, and carp) and genetic monogamy is extremely rare in the marine environment. It is thus safe to say that diploidy and genetic monogamy are not good friends, at least not in any case where the fish is not sterile. Knowing how important this issue is, let’s take a closer look at how much chromosomes fish actually have, how they are organized, and how this all relates to their evolution.

Fishes’ Karyotype Is Most Commonly Triaploid (3n)

In vertebrates, the number of chromosomes associated with a particular species is referred to as the karyotype. As in humans, the karyotype of fish is usually represented by a pair of haploid sets of chromosomes (n = 22 in most cases), resulting in a total chromosome number of 3 (triploidy). Although rare instances of tetraploidy are known, these are thought to be the result of chromosome duplication (e.g., whole-chromosome duplication followed by partial chromosome loss). Whole-chromosome duplication is thus extremely rare in fishes, with the exception of the giant African pouched rat, which has successfully used this strategy to increase the size of its chromosomes (from n = 14 to n = 28). Fishes with the highest chromosome number ever recorded are the three-spined stickleback, with n = 72. This is due to the whole-chromosome duplication followed by partial chromosome loss that generated a diploid set of 48 chromosomes in this species (2n = 24; 3n = 24). Most fishes have a karyotype made up of two sets of haploid chromosomes (e.g., 2n or 3n), but there are some notable exceptions, such as the Japanese medaka, which has a tetraploid karyotype made up of four sets of haploid chromosomes (4n = 28). It should be emphasized that the term karyotype does not necessarily imply that all the chromosomes of a given organism are the same, nor that they carry the same genetic information. It simply means that a certain number of chromosomes have been identified and counted in a particular species.

How Many Chromosomes Does Fish Have?

The number of chromosomes is often expressed as a multiple of two (e.g., 2n, 4n, 6n, etc.). The karyotype of most vertebrates is made up of a total number of chromosomes that is a multiple of two. This is why, when talking about the karyotype of fish, it is usually expressed as 3n. Thus, fishes have three times as many chromosomes as the haploid number that would be expected in that species. This is also why most fish are known as diploid or haploid organisms, as they have only two or three chromosome sets. There are, however, about 20 exceptions to this rule, with the majority of these species having a karyotype made up of four sets of chromosomes (e.g., 4n). The only other case of a fish with a tetraploid karyotype is the Japanese medaka, and even in this case, it is believed that the increase in chromosome number was due to whole-chromosome duplication followed by partial chromosome loss. What this means is that, in all other cases of fishes, two sets of chromosomes are considered to be the basic unit of all the chromosomes in a given organism. It should be emphasized that this is not always true; there are some species of fish that have a significant amount of material spanning over more than two chromosome sets (e.g., the blackmouth seabream, with 18 pairs of chromosomes in the third division of the genome). This is why the number of chromosomes in a fish is such an important factor to consider, as it can provide valuable insights into the basic genomic organization and evolution of that species, as well as the mechanisms of chromosome inheritance and loss, which may have occurred in the past. Due to its vast importance, and since it is generally agreed that the majority of fishes are diploid, the haploid number is used as a reference point when discussing the karyotype of fish.

Chromosomes Are Placed In Specific Orders & Locations On The Fish’s Body

When we look at the karyotype of a fish, we can usually identify a basic pattern, where the chromosomes are arranged in a specific order and located in specific regions of the body. This basic pattern is known as the โ€˜structure of the chromosome’ and may vary slightly from one species to another. From the basics to the specifics, here are some of the most important things to keep in mind:

The Majority Of The Fish Genome Is Located On The Pairs Of Sex Chromosomes (Z & W)

When we look at the basic organization of the chromosome in a fish, we can usually see that the majority of the genome is located on the pairs of sex chromosomes. This is particularly easy to spot in cases where the chromosomes have been arranged in columns, as in the platyfish and the medaka. In most cases, these are the two largest chromosomes in the genome, with the Z chromosome being the female sex chromosome and the W being the male sex chromosome. The exceptions to this are the stickleback, where the W chromosome is smaller than the Z chromosome, and the bony fishes, where the Z chromosome is actually smaller than the W chromosome. In these two cases, the rest of the genome is made up of the smaller, acrocentric chromosomes (see Figure 1).

The Z & W Chromosomes Are Located At The Ends Of The Chromosomes

One of the most important things to keep in mind when looking at the karyotype of fish is that the Z and W chromosomes are typically located at the ends of the chromosomes. This is why, when looking at the chromosomes of fish in metaphase, these are the two largest and most prominent chromosomes. The exceptions are the platyfish and the medaka, where the W chromosome is also the longest chromosome. When this is the case, it becomes difficult to identify which chromosome is which, as both look like giant centromeres (see Figure 2).

The Chromosomes Have Specific Naming Conventions

All the chromosomes in a fish have specific naming conventions, which help us identify them more easily. One of the most important things to keep in mind is that the fish chromosomes are different from the ones in mammals. The primary difference is that the chromosomes are not named according to size, like the chromosomes in mammals. Instead, fish chromosomes are named after the proteins that are encoded by them. For example, the largest chromosome in the platyfish is called the pachytene chromosome and the smallest chromosome in the bony fish is called the microchromosome. It should be noted that this is not always the case and that some chromosome pairs, such as the Z and W chromosomes, follow the same naming convention as the one described above. It is also important to remember that some of the smaller chromosomes, such as the microchromosomes in the bony fish examples, can be the same size as some of the largest ones. This means that it can be difficult to tell which chromosome is which, especially when there is a large amount of material on the slide (see Figure 3).

Craving More Content?

Fishy Features