Barriers to Hybridization And Sterility of Interspecific Hybrids as Mechanisms Preventing the Mixing of Alleles Between Species
- Barriers to hybridization are mechanisms that prevent different species from mating and producing viable, fertile offspring.
- These barriers maintain the genetic distinctness of species by reducing or eliminating gene flow between them.
Gene flow
The movement of genes from one population to another
What Are Barriers to Hybridization?
- Barriers can be classified into two types:
- Prezygotic Barriers: Prevent mating or fertilization.
- Postzygotic Barriers: Prevent hybrid offspring from surviving or reproducing.
- Each play a vital role in preserving species boundaries and biodiversity.
Prezygotic Barriers: Preventing Fertilization
Prezygotic barriers
Prezygotic barriers act before fertilization, stopping different species from mating or ensuring that fertilization is unsuccessful.
1. Temporal Isolation
Species reproduce at different times of the day, season, or year.
ExampleTwo populations of pine processionary moths in Portugal breed in different seasons, preventing interbreeding.
2. Behavioral Isolation
Differences in courtship behaviors or mating rituals prevent interbreeding.
ExampleBirds of paradise perform species-specific courtship displays that ensure individuals recognize suitable mates of their own species.
3. Mechanical Isolation
Differences in reproductive anatomy make mating physically impossible.
ExampleInsects often have species-specific genital structures that ensure reproductive compatibility only with their own species.
4. Gametic Isolation
Even if mating occurs, sperm and egg cells of different species may be incompatible, preventing fertilization.
ExampleIn sea urchins, surface proteins on eggs and sperm must match for successful fertilization.
Postzygotic Barriers: Preventing Viable or Fertile Offspring
Postzygotic barriers
Postzygotic barriers occur after fertilization, ensuring that hybrid offspring are inviable or sterile.
1. Hybrid Inviability
Hybrid embryos fail to develop properly, often due to genetic incompatibilities.
Example- The mule, a sterile hybrid of a horse (Equus caballus) and a donkey (Equus asinus), combines the strength of a horse with the hardiness of a donkey.
- However, chromosomal mismatches during meiosis prevent mules from reproducing, ensuring no further mixing of horse and donkey alleles.
2. Hybrid Sterility
Hybrids are unable to reproduce because their chromosomes cannot align correctly during meiosis.
ExampleMules (horse × donkey hybrids) are sterile due to having 63 chromosomes, an uneven number that prevents proper pairing during gamete formation.
3. Hybrid Breakdown
Even if hybrids are fertile, their offspring may be weak or sterile, reducing their fitness over generations.
ExampleSome hybrid plants produce fertile offspring, but subsequent generations show reduced viability.
Why Is Hybrid Sterility Important?
- Hybrid sterility acts as a safeguard against wasted reproductive effort.
- Producing sterile hybrids is an evolutionary dead-end because sterile individuals cannot pass on their genes.
- This drives natural selection to favor mechanisms that prevent hybridization altogether.
Genetic Incompatibility in Meiosis
- The sterility of hybrids often arises from mismatched chromosome numbers or structures.
- During meiosis, homologous chromosomes fail to align or segregate properly, resulting in nonviable gametes.
Natural and Artificial Hybridization
Natural Hybridization
- Hybridization occurs in nature when closely related species overlap in range.
- However, hybrids are often sterile or inviable, limiting gene flow.
- Hawaiian ducks (Anas wyvilliana) are threatened by hybridization with introduced mallards (Anas platyrhynchos),
- This forms hybrid swarms that reduce the genetic identity of the native ducks.
Artificial Hybridization
- Humans have intentionally bred hybrids to combine desirable traits from different species.
- These hybrids are often sterile, ensuring they do not contribute to natural gene pools.
In plants, hybrid sterility is less common because they can often reproduce asexually or through polyploidy (doubling their chromosome number), which can restore fertility.
The Role of Barriers in Maintaining Biodiversity
- Barriers to hybridization are essential for preserving species boundaries and biodiversity.
- Without these mechanisms:
- Gene Pools Would Mix: Species would lose their distinct genetic identities.
- Ecological Roles Would Blur: Specialized adaptations might be lost, reducing ecosystem efficiency.
- Extinction Risks Could Rise: Hybrid swarms might replace native species, as seen in Hawaiian ducks.
When Barriers Fail
- While hybridization often poses challenges to biodiversity, it can sometimes drive speciation.
In plants, for instance, polyploidy (doubling of chromosome sets) can create new, fertile species from hybrids.
TipTip:In plants, breeders often induce polyploidy to restore fertility in hybrids, creating new species with traits from both parent species.
Self review- What are the key differences between prezygotic and postzygotic barriers?
- Why are mules sterile, and how does this relate to chromosome pairing during meiosis?
- How does courtship behavior act as a mechanism to prevent hybridization?
