species and reproductive isolation

Good day to everyone and welcome to new session of writing from me after what seemed like aeons. As some of you already know, I could not blog for some period because of issues associated with my laptop. I am glad to inform you all now that the issues have been fixed and hopefully, I will have time to resume regular session of blogging again - starting with today.

One of the first few things to learn in high school biology is taxonomy. That is the science of classifying living organisms. According to Carl Linnaeus, the man generally referred to as the father of taxonomy, living organisms are classified and grouped into various hierarchical levels based on certain similarities or relationship. Hence, living organisms were classified into different kingdoms, phyla, classes, orders, families, genus, and species. However, today's write-up is not about taxonomy per se, I will be taking a closer look at the lowest taxonomic category: that is the species. In case any of my audience is interested in reading more about taxonomy in general, I will recommend this link.

^{By Sumita Roy Dutta (Wikimedia Commons ) - Own workCC BY-SA 3.0, Link}

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The concept of species

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The 'species' occupies the lowest rung of the taxonomic ladder as far as classification of living organisms is concerned. Organisms classified as the same species share a lot of characteristics, including morphological, behavioural, genetic biochemical among other features. A closer look at the definition of species throws up an interesting subject that is worth discussing. Simply put:

species is a group of individuals capable of producing fertile offsprings through breeding or mating.

There are billions of plants and animal species on earth along with species of organisms in other kingdoms such as protista, monera, and fungi. In earnest, what qualifies two organisms as same species is their ability to produce fertile offsprings through mating. Nature has a way of reproductively isolating each species of organism in order to ensure that inter-species breeding does not take place and if it does takes place, the resulting offspring will appear infertile. For example, humans have been reported and are known to engage in sexual activities with animals such as horses, dogs, apes etc but have never been able to produce human/animal hybrid. The closest that scientists have come to producing inter-species hybrid yielded infertile offsprings as in the case of a mule which is a horse/donkey hybrid, liger/in which are tiger/lion hybrids among many several other examples.

However, exceptions usually occur when breeding takes place between closely related species. Example of such is found in beefalo which is a hybrid of cattle/buffalo and wolfdogs which are hybrids of wolves and dogs. Generally, it is extremely rare but scientifically possible for inter-species hybrids to be fertile, especially in closely related species as cited in the above examples. This is simply because “Species” is a term meant to draw a line around a group with no specific agreement about where exactly such line is to be drawn.

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The process of fertilization

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In sexually reproducing animals living organisms, the male produces haploid sperms through meiosis while the females produces eggs through the same means. These two gametes fuse together in a process known as fertilization to form a diploid zygote. In animals, the zygote through repeated cell division and embryogenesis eventually develops to become a new organism while a new fruit is formed in plants. These processes only happen in reproductively compatible individuals, that is individual of the same species. What if sperms and eggs from different species of organims meet during fertilization process? In order to completely understand the answer to this question, let us have a closer look at the structure of human sperm and the egg and what usually happens during fertilization.

There are basically three components of the male sperm, the head which is made up of the acrosome, the cell membrane and the nucleus; the middle piece which contains filamentous core housing several mitochondria and the tail. The acrosome of the head houses the enzyme needed for penetrating the egg during fertilization while the nucleus contains all the genetic information in form of densely coiled chromatin fibres. It also contains organelles like the vacuole and the centriole. The mitochondria in the filamentous core of the middle piece function in the synthesis of ATP, the energy needed for the movement of the spermatocytes through the female cervix, uterus and uterine tubes while the tail is used like a paddle in the movement of the spermatocyte.

The ovum of females is is surrounded by three main envelopes including the vitelline membrane which is inner, thin, transparent and is secreted by ovum itself, the zona pellucid which is located at the middle, thick, transparent and non-cellular secreted partly by follicular cells and partly by the oocyte. The third is the corona radiate which is outer and thicker coat formed of radially elongated follicular cells. In between the vitelline membrane and zona pellucid lies a narrow perivitelline space. Within the ovum lies the cytoplasm which is well differentiated into transparent exoplasm or egg cortex and opaque endoplasm or ooplasm. The nucleus is usually large and commonly referred to as the germinal layer.

During fertilization processes, a sperm that will fertilize an ovum needs to pass through three barriers in order to reach ooplasm. These barriers include the jelly coat surrounding the ovum, the vitelline membrane together with the zona pellucid and of course the plasma membrane. The sperm first attaches itself tp the jelly coat of the ovum while the acrosome cap which usually contains digestive enzymes that is used in eating away the jelly layer of the ovum. Thereafter, the sperm reaches the vitelline layer and eventually fuses with the plasma membrane. At this junction, the sperm's nucleus enters the cytoplasm where it fuses the nucleus of the egg to form a zygote. A process is initiated during these fertilization processes which serves to prevent a situation of fusion of more than one sperm with an egg. This process is known as cortical reaction. Anyone interested in what happens during this process can read more here

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So what happens if the sperm and egg of different species meet?

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Like I said earlier, mating between two different species is not actually the issue, the issue is that often times, the mating results in no fertilization or formation of infertile offsprings and that only cases of mating between two closely related species can result in production of fertile offsprings.

In order to answer the question above, three events are possible if the sperm and egg of different species meet;

• No fertilization
• Fertilization, but resulting offspring becomes sterile
• Fertilization and resulting offspring normal.

In the first event, the vitelline layers prevents the sperm from getting access to the plasma membrane of the egg thereby preventing fusion from taking place. The layer is a species-specific boundary that is actively involved in sperm-egg recognition. Therefore, the layer has recognition mechanism that only permit sperm of same or similar species to pass through and get access to the plasma membrane of the egg.

Sometimes, the vitelline layer mistakes sperms from another species as being of same species with the egg and allows such sperms to go through and get attached to the plasma membrane of the egg resulting in fertilization. Such fertilization if not deleterious usually result in an invalid or sterile offsprings.

Two species even though different might have close phylogenetical relationship with each other. Mating between such species will results in normal fertilization and the resulting offsprings usually appear normal and fertile. Such is in the case of the beefalo and the wolfdog.

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Other methods of reproductive isolation

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The species-specific nature of the vitelline layer of the female ovum is one of the methods of reproductive isolation. By reproductive isolation, we are simply referring to various evolutionary mechanisms necessary for the process of speciation. These mechanisms could be before the zygote is formed otherwise known as pre-zygotic reproductive isolation mechanisms or after the zygotes have been formed otherwise known as post-zygotic reproductive isolation mechanisms. While the former include mechanisms such as gametic isolation as in the case of the vitelline layer of the ovum, behavioural isolation, temporal isolation and mechanical isolation, the latter include zygote mortality or non-viable hybrid and hybrid sterility. All these processes ensure that populations of organisms evolve into distinct species.

So next time you see species of different organism, you have an idea of went went down.

Thank you for reading.

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Text References

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^species

^{is cross fertilization possible?}

^sperm

^{structure of human sex gametes}

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