Interspecific hybrids show variable degrees of sterility.

Understanding Interspecific Hybridization and Sterility

Interspecific hybridization aims to combine favorable traits from distinct species. However, the genetic divergence between species often leads to reproductive isolation, manifesting as sterility in the hybrid offspring. The degree of sterility varies considerably – some hybrids are completely sterile, while others exhibit partial or even complete fertility. This variability is a result of complex interactions at the chromosomal and genetic levels.

Mechanisms Leading to Sterility in Interspecific Hybrids

• Chromosome Number and Structure Differences: Species often differ in chromosome number (ploidy) and structure. During meiosis, proper chromosome pairing is crucial for successful gamete formation. In interspecific hybrids, unequal chromosome numbers or structural differences (translocations, inversions) can disrupt pairing, leading to unbalanced gametes and sterility. For example, crosses between Triticum boeoticum (2n=14) and Aegilops squarrosa (2n=14) can produce sterile hybrids due to irregular meiosis.
• Genetic Incompatibilities (Cytoplasmic and Nuclear): These are genes that interact negatively when inherited from different species, disrupting development or fertility. Cytoplasmic male sterility (CMS) systems, where the cytoplasm from one species inhibits pollen development, are a common example. Nuclear genes can also interact negatively, affecting embryo development or other reproductive processes.
• Genetic Segregation: Incomplete linkage between genes from different species can lead to unfavorable gene combinations in the hybrid progeny, impacting viability and fertility.

Variability in Sterility Degrees

The degree of sterility isn't uniform; it's influenced by several factors:

• Genetic Distance: Hybrids between closely related species are more likely to exhibit partial or complete fertility compared to those between distantly related species.
• Chromosome Pairing Ability: The ability of chromosomes from different species to pair during meiosis significantly impacts sterility. If pairing is relatively normal, fertility might be higher.
• Cytoplasmic Factors: The presence or absence of cytoplasmic factors affecting pollen development plays a crucial role.
• Selection and Breeding: Repeated backcrossing and selection can sometimes improve fertility in initially sterile hybrids, although this is challenging.

Strategies to Overcome Sterility

While complete fertility restoration is often difficult, breeders employ various strategies:

• Chromosome Balancing: Techniques like induced polyploidy (e.g., doubling chromosome number) can sometimes equalize chromosome numbers and facilitate pairing.
• Bridge Organisms: Using intermediate species that can bridge the genetic gap between the target species can sometimes lead to more fertile hybrids.
• Genetic Engineering: Modern techniques like CRISPR-Cas9 can be used to modify genes involved in sterility, although this raises ethical considerations.

Case Study: Durum Wheat and Wild Relatives

A significant challenge in durum wheat (Triticum durum) breeding is incorporating disease resistance genes from wild relatives. Crosses with Triticum dicoccoides often produce sterile or semi-sterile hybrids due to chromosome incompatibilities. Researchers are using techniques like embryo rescue and chromosome doubling to overcome this sterility and recover fertile plants carrying desirable genes.

Species 1	Species 2	Sterility Degree
Triticum durum	Triticum dicoccoides	High (often sterile)
Hordeum vulgare	Hordeum arundinaceum	Variable (moderate to high)
Oryza sativa	Oryza rufipogon	Variable (can be reduced with selection)