When different species mate, the resulting offspring are referred to as hybrids. While hybridisation can occur, nature has erected various barriers to prevent it. Even when hybrids are formed, they often confront the challenge of sterility. This segment delves deeper into these fascinating biological phenomena.
Barriers Preventing Hybridisation
Interspecific hybridisation denotes the breeding between two distinct species. Multiple barriers can curtail such hybridisation events, ensuring that species maintain their uniqueness.
Prezygotic Barriers
These barriers operate before the fertilisation of the ovum:
- Temporal Isolation: Different species often have distinct breeding periods. This could be different times of the day or distinct seasons. For instance, two species of frogs living in the same pond might breed at different times of the year, reducing the chances of hybridisation.
- Behavioural Isolation: Many species have unique mating rituals or calls. These rituals can be so distinct that they don't attract members of other species. For instance, firefly species often have differing flash patterns, ensuring they attract the correct mates.
- Habitat Isolation: Even if two species live in close proximity, if they inhabit different habitats, they might never encounter each other. For instance, one insect species might live on the forest floor while another inhabits the canopy.
- Mechanical Isolation: This relates to the physical differences in reproductive organs of different species, preventing successful mating. For instance, flowers of different species might be adapted to pollination by different types of insects due to the size or depth of the flower.
- Gametic Isolation: Even if mating happens, the gametes (sperm and egg) of two different species might be unable to fuse. This can be due to chemical incompatibilities, differences in the number of chromosomes, or other genetic factors.
Postzygotic Barriers
These barriers come into play after fertilisation:
- Hybrid Inviability: After fertilisation, the embryo might not develop correctly. This can be due to genetic incompatibilities that prevent the embryo from growing or reaching maturity.
- Hybrid Sterility: Sometimes a hybrid can grow to adulthood but will be unable to produce offspring. This is often seen when there's a mismatch in the number of chromosomes from each parent.
- Hybrid Breakdown: In some cases, first-generation hybrids might be viable and fertile, but when they mate, their offspring (the second generation) are weak or sterile.
Sterility of Interspecific Hybrids
The inability of a hybrid to reproduce is termed sterility. This is a frequent outcome when two different species mate. Sterility ensures that the gene pools of the two parent species remain distinct.
Why are hybrids often sterile?
- Mismatched Chromosomes: If the parent species have different numbers of chromosomes or structurally distinct chromosomes, it can impede the hybrid's ability to produce balanced gametes. For instance, if one parent species has 14 chromosomes and another has 16, the hybrid might inherit 15. This odd number makes meiosis, the process of forming gametes, problematic.
- Gene Interactions: Genes from one species might produce products that interact harmfully with those from the other species. This can disturb the normal development or function of reproductive organs in the hybrid.
Examples of Sterile Hybrids
- Mules: A classic example is the mule, a hybrid between a male donkey and a female horse. Mules inherit 63 chromosomes, a mix between the horse's 64 and the donkey's 62. This mismatch prevents mules from producing balanced gametes, leading to sterility.
A Chestnut Mule (Equus ferus caballus) in Israel.
Image courtesy of SuperJew
- Liger: A cross between a male lion and a female tiger. Similar to mules, ligers face reproductive challenges due to chromosome mismatches.
Liger in Shenzhen Safari Park, Shenzhen, China.
Image courtesy of Dinkun Chen
- Hinny: Less commonly known than the mule, a hinny is the offspring of a male horse and a female donkey. Like mules, hinnies are also typically sterile due to chromosome disparities.
Hinny
Image courtesy of Tsaag Valren
FAQ
Yes, there are instances where hybrids are fertile and can reproduce. For example, the European red deer and the sika deer have produced hybrids that are often fertile. Another instance is the hybrid canary, which can breed with either of its parent species. However, it's worth noting that while fertile hybrids do exist, they're relatively rare in nature. The fertility of these hybrids can sometimes lead to genetic mixing and even the formation of new species, especially if they have a reproductive advantage in their environment or if they can exploit a niche not occupied by their parent species.
In plants, one fascinating mechanism to overcome hybrid sterility is polyploidy. Polyploidy refers to the condition where an organism has more than two complete sets of chromosomes. If a hybrid plant is sterile due to unmatched chromosome sets from its parents, a spontaneous duplication of its entire chromosome set can occur, resulting in a polyploid individual. This polyploid plant can then produce gametes normally and reproduce. Over time, these polyploid plants can give rise to a new species that's distinct from the parent species. Such events have played a significant role in plant evolution and diversification.
Apart from sterility, genetic incompatibility in hybrids can manifest in various ways. Some hybrids may exhibit poor health, reduced vigour, or shorter lifespan. They might also show morphological abnormalities, like distorted body structures or irregular patterns. In plants, hybrid offspring might display irregular growth patterns or be more susceptible to diseases and pests. Such manifestations result from the genetic conflicts between the two sets of chromosomes inherited from the different parent species. In essence, these genetic discrepancies disrupt the normal developmental or physiological processes of the hybrids.
Human intervention, especially with advanced breeding techniques and genetic engineering, can sometimes produce fertile hybrids that wouldn't arise in nature. By manipulating the environment, selecting for specific traits, or even directly altering the genetic makeup, humans can influence hybrid outcomes. However, creating fertile hybrids still hinges on the genetic compatibility of the species involved. While science has made significant strides, there are inherent genetic barriers that can't always be bypassed easily. Nonetheless, in agriculture and horticulture, human intervention has successfully produced numerous hybrid varieties that benefit from the combined traits of their parent species.
Barriers to hybridisation have evolved primarily to maintain species integrity. The presence of such barriers ensures that the gene pool of each species remains distinct, preserving unique adaptations and characteristics. If hybrids between species were common and fertile, the distinctions between species could blur, leading to a loss of biodiversity. The barriers also reduce the chances of producing offspring with lower fitness. Hybrids, especially in the wild, might not be as well-suited to their environment as pure species, which could reduce their chances of survival and reproduction. In essence, these barriers promote the long-term survival and adaptation of species.
Practice Questions
Prezygotic barriers act before the fertilisation of the ovum, ensuring that fertilisation does not occur between species. Two examples include temporal isolation, where species breed at different times, and behavioural isolation, where species have unique mating rituals that are not recognised by other species. On the other hand, postzygotic barriers operate after fertilisation and often result in the hybrid offspring being unable to reproduce or survive. Hybrid inviability is one such barrier where the hybrid does not survive to reproductive age. Another is hybrid sterility, where the hybrid reaches adulthood but is incapable of producing offspring.
Interspecific hybrids like mules and ligers are often sterile due to chromosome mismatches between the parent species. Mules, for instance, result from the breeding between a male donkey and a female horse. Donkeys have 62 chromosomes, and horses have 64, so mules inherit an intermediate total of 63 chromosomes. This uneven number makes the process of meiosis, which forms gametes for reproduction, problematic. Similarly, ligers, offspring of a male lion and a female tiger, face reproductive challenges because of disparities in the number of chromosomes inherited from each parent. The inability to produce balanced gametes due to these mismatches leads to their sterility.
