Cyprinodontiformes vivíparos e ovovivíparos

Livebearer Cyprinodontiformes

Biologia > Problemas genéticos em populações reduzidas

Biology > Genetic problems in small populations

Genetic problems in small populations

When wild populations of certain species in nature reach extremely low number of individuals faces imminent danger of extinction.

Nowadays in the presence of the demographic limitative factors, the only approach to save living creatures is to bring them into captivity under a breeding program in controlled environment, hoping this way build up the effective’s number, and try to re-establish the wild population on natural habitat later.

In captivity urgent actions are imperative to maintain reproductive viability, in particular among species facing threat or danger of extinction in their original ecosystems. It becomes necessary invest in a process to reach auto sustained level for a population under this conditions can survival.

On a different stage, very far away from domestic aquarium fish, the process is being tried with endangered species, but it involves many challenges, both biological and political.

Private fish keeping rarely can put together all the proper conditions and space to a population sustainable level, unless cooperation between quite a few holders of the same specie.

Several problems arise when a species reaches a very small population size, specially this :

1) Loss of genetic variability

Depending on the specie we are talking about; when a fish population drops below a number with reference to something like 100 breeding individuals, the amount of genetic dissimilarity is reduced, and these can lead to eventual diminish of immunity capacity as to adaptation to environmental changes.

Some livebearer Cyprinodontiformes are genetically extremely uniform. 

This indicates that they are hereditarily identical as far as genes are concerned, even if they are not relatives.

The immune systems of all animals are absolutely dependent on genetic diversity.

There are probably millions of genes linked to immunity, each one coded for a specific antibody or cell surface receptor. 

The problem arises when there isn’t enough information about these genes at chromosomes.

Small groups raised in aquarium are not the only cause to loss of genetic variability.

When somewhere at an evolutionary process an event outcomes and a significant percentage of certain specie is killed, or otherwise prevented from reproducing, the reduced survival or breeding group is facing what scientists call a population bottleneck or genetic bottleneck.  

At this moment the sustainability of that specie will be deeply affected.

Genetic variability can often be evaluated by gene diversity ( allelic richness and diversity, but also heterozygosity ).

2) Inbreeding

Populations below about 50 breeding individuals are forced to breed with close relatives.

The pattern of breeding between genetically so close fish is called inbreeding.

As end result, this can lead to depression of fitness called inbreeding depression, which includes lethal consequences to that population beginning with decreased fertility and fecundity, high mortality rate and birth defects.

Aquarium populations which went down to less than 30 individuals, have to be managed very carefully in order to minimize inbreeding deadly effects.

In certain captive populations may well show inbreeding symptoms, even among non-related fish, precisely because some Livebearer Cyprinodontiformes are indeed genetically very homogeneous by nature and are more vulnerable to reduced populations keeping.

Negative impacts from lack of diversity can increase genetically transmitted diseases, physical decrepitude, wide-ranging debility, and undersized adult individuals.

It is almost impossible to determine how far reduced populations keeping will affect particular species, as there is more or less susceptibility to this problem from one to another.

3) Selective breeding and induced hybridizing

When under man made breeding selection many species are equally submitted to loss of genetic variability but above all to inbreeding.

Searching for improvement or strengthening of specific characters, there are significant risks of genetically transmitted anomalies to be reinforced on the process in the same way. 

With long periods in captivity there is a genuine danger that unintentional selective breeding can result in the establishment of traits that are desirable in captivity but opposed to natural selection.

Aquarium conservation may be seen as apparently the most viable way to save in danger of extinction species in the so called « ex situ conservation », but unnatural strain selection may endanger definitively this last survival members before an eventual return to the wild.

Besides man made selection to obtain a specific strain, forced hybridizing must be considered as risk factor.

Take for an example, that endangered species of which all the remaining individuals of the same sex were crossed with what was thought to be their closest relative.

Theoreticaly it was possible to produce hybrids that, could be interbred to give back a strain of the threatened species that could stay close to 100% “ pure “ of the original. 

Unfortunately, almost all this events have failed either by bad choices on the genetically closest specie but also by unpredictability of the genetic course.

Natural hybridization seems to be unusual. However it con occur oftenly when species are reduced to very small numbers.

Non natural fertil hybrids in the wild can become a menace to parental species biodiversity and survival as independent forms of live. 

Hybrid vigor can soon overcome breeding chances wile competing with members of original species.