Chromosome Mismatch Causes Hybrid Sterility
- Chromosomes are the carriers of genetic information, organized in pairs in sexually reproducing organisms.
- During reproduction:
- Gametes (sperm and egg cells) are formed via meiosis, which reduces the chromosome number by half.
- For meiosis to succeed, homologous chromosomes (matching pairs inherited from each parent) must align and segregate properly.
- When two species with different chromosome numbers interbreed, their hybrid offspring may inherit an uneven chromosome set.
- A classic example is the mule, a hybrid between:
- Horses: 64 chromosomes.
- Donkeys: 62 chromosomes.
- Mules inherit 63 chromosomes, which cannot form homologous pairs.
- As a result:
- Meiosis fails.
- Functional gametes cannot form, rendering mules sterile.
- It’s a common misconception that all hybrids are sterile.
- While this is often true, there are exceptions.
- For example, some female ligers (lion × tiger hybrids) can produce offspring.
Plants Are a Special Case
- Unlike animals, many plants can bypass these issues through polyploidy.
- This is where entire sets of chromosomes are duplicated.
- Polyploidy allows:
- Homologous pairing of chromosomes.
- Fertility in hybrids.
Definition
Polyploidy
Polyploidy is the duplication of an organism’s entire chromosome set.
Polyploidy in animals is rare because their cells are more sensitive to changes in chromosome number, whereas plant cells are more adaptable.
Why Chromosome Number is a Shared Trait
- Within a species, having the same chromosome number ensures:
- Viable gamete formation through homologous pairing in meiosis.
- Genetic stability across generations.
- Fertility of offspring, maintaining the species as a coherent unit.
Speciation and Chromosome Number
- Over evolutionary time, chromosome number can change through fusion, fission, or duplication events.
- When populations diverge and accumulate differences in chromosome number, interbreeding becomes impossible.
- This acts as a mechanism of reproductive isolation, driving speciation.
- When explaining why hybrids are infertile, always mention failure of homologous pairing in meiosis.
- This is the key mechanism expected in IB answers.
- Why do all members of a species typically share the same chromosome number?
- Explain why hybrids with mismatched chromosome numbers are often infertile.
- How can changes in chromosome number contribute to the process of speciation?
