The following points highlight the top two modes of reproduction in animals. The modes are: 1. Asexual Reproduction 2. Sexual Reproduction.
Mode # 1. Asexual Reproduction:
It is most common in lower animals but occurs also in somewhat highly organised forms. Production of a generation of cercaria from a redia in liver fluke, budding in bryozoa and tunicates belong to this category.
In asexual reproduction the sexual units, male and female gametes are not involved. A single individual is capable to produce offspring’s. Binary fission, multiple fission and budding are the common modes of asexual reproduction.
Binary fission:
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It may be transverse, longitudinal and oblique. The individual divides into two equal halves, each half grows into an adult individual. The nucleus divides mitotically into two, which is followed by a division of the cytoplasm, each half receiving a daughter nucleus.
In transverse fission, the cytoplasm divides by a transverse constriction, e.g. Amoeba, Paramoecium, etc. In longitudinal fission the split appears at the anterior end and proceeds backward, e.g. Vorticella, Euglena, Trypanosoma, etc.
The half lacking some asymmetrical structures constructs them and a full grown adult is formed. Oblique fission is similar to that of transverse fission but the cytoplasm divides obliquely, e.g. Dinoflagellates, Ceration, etc.
Multiple fission:
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It is also known as sporulation. In sporulation the nucleus undergoes repeated mitotic divisions producing a large number of daughter nuclei. The cytoplasm breaks up into many fragments, a bit surrounds a daughter nucleus and a tiny individual is formed.
On liberation from the covering, the individuals grow and reach the adult stage. Multiple fission is the normal mode of reproduction in sporozoa. In unfavorable conditions Amoeba undergoes multiple fission.
Budding:
Multiplication by budding involves formation of gemmules or internal buds in sponges and lateral buds in some invertebrates and chordates, viz. sponges, cnidarians, bryozoa, tunicates and others. A lateral bud is given off from some part of the parent animal, it grows and assumes the shape of a complete parent animal and separated from the parent either before or after completion of the development.
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In some cases, the fully developed buds remain in a vital continuity with the parent and form a compound animal consisting of a number of units. Such a compound animal is termed a colony, e.g. Volvox, Obelia, Bugula, etc..
Mode # 2. Sexual Reproduction:
In sexual reproduction, two sexual units or gametes are involved. Prior to the formation of gametes the germ cells undergo a pre-gametic meiosis and the chromosome number is reduced to half (n) in the gametes.
The original number (2n) is restored in the zygote by the union of two gametes, which produces a new individual. The gametes may be similar and the union is known as isogamy or they may be dimorphic or dissimilar and the union is named as heterogamy.
The sexual reproduction is much more advantageous than asexual reproduction. Formation of germ cells, which are small in size, not only economise the involvement of living materials but also less exhausting physiologically.
More important than these is the union of two units, coming from different sources, bringing different materials, and their combination is a perennial source of variations, which are the raw materials for evolution.
Majority of the animals are male or female, and the evolution of sex occurred in the early period of animal history. The male products are tiny, nearly always motile spermatozoa and the female products are fairly large, passive ova often containing yolk.
Sexual reproduction occurs in all groups of animals. Starting from protozoon it becomes more and more complex, attaining perfection in mammals. Sexual reproduction is of two types, conjugation and fertilization.
Conjugation:
Three types of conjugations are found in animals:
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Simple conjugation:
It is the simplest form of sexual reproduction. Two similar cells unite with the fusion of their nuclei, e.g. rhizopods, sporozoa, flagellates and other protozoa. The conjugation is temporary in certain ciliates, Paramoecium, for exchange of nuclear materials.
Multiple conjugation:
More than two cells unite temporarily, as in some sporozoa.
Dimorphic conjugation:
Two cells of different sizes fuse to form an individual. In Vorticella, the small active, free swimming unit (male) fuses with a normal size fixed individual (female).
Fertilization:
Fusion of a microgamete with a macrogamete. Found in many Protozoa, Eimeria, Plasmodium, etc. More commonly, the male gametes or spermatozoa are formed in the male gonads or testes and the female gametes or ova in the ovaries of females in higher forms. Only one spermatozoon fuses with an ovum to form a zygote, the unicellular starting point of multicellular life
Uni-sexuality and bisexuality:
A range of modifications occur in the invertebrates in the location, arrangement, number and types of gonads; their ducts and other accessory reproductive organs. Depending on the presence of one or both types of gonads they have been placed in two groups.
Unisexual or dioecious:
The individual possesses either the testes or the ovaries.
Examples:
Round worms, bivalves, prosobranchs, cephalopods, vertebrates, etc.
Bisexual or hermaphrodite or monoecious:
The individual possesses both the testes and the ovaries.
Examples:
Sponges, Hydra, annelids, barnacles, pulmonate molluscs, etc. In majority of the bisexual animals, however, self-fertilization is prevented by producing sperms and ova at different periods of time (dichogamy). Though rarely, self-fertilization (autogamy) occurs in some parasitic flatworms.
Protandry:
In some animals the same gonad first produces male gametes and then female gametes. This is known as protandry and the animal protandrous. Examples: Some nematodes, bivalves, gastropods, star fishes, tunicates, cyclostomes, etc.
Protogyny:
The first female and then male condition is known as protogyny and the animals protogynous. Protogynous forms are rare and occur only in some fishes.
Invertebrate reproductive system:
A range of diversity is found in the components of the reproductive system in invertebrates. The gonads are temporary structures in some diploblastic animals and polychaetes.
They are derived from amoebocytes, a type of mesenchyme cells in sponges; both from ectoderm (Hydra) and endoderm (Aurelia) in cnidaria and the gametes are shed outside or in the radial canals respectively. In all triploblastic forms gonads are mesodermal in origin.
The number of testes in an individual varies greatly. They are one pair in most flatworms, polychaetes, arthropods and molluscs; 2 pairs in some cnidarians (Obelia, Aurelia), earthworms; 5 to many pairs in leeches and only one in round worms and gastropod molluscs. In the latter, one of the testes disappears during torsion in the embryo.
The number of ovaries is more constant in the invertebrates. They are two pairs in some cnidarians (Obelia, Aurelia), one pair in roundworms, annelids (except nereis, where the ovaries are many and temporary), arthropods and one in flat worms and gastropods. One ovary degenerates during torsion in gastropods.
Both the testes and ovaries are connected with ducts. In males, the sperms are discharged to the exterior through a pair or one pore. In females, the ova are carried to a particular site in the body for fertilization or released to the exterior through a pore. Certain glands and specialised structures constitute a part of the reproductive systems of both males and females.
Vertebrate reproductive system:
In all craniata the gonads are mesodermal in origin, paired and located in the coelom, suspended to the dorsal wall by a fold of peritoneum. The single pair of gonads develop from the dorsal wall of coelom as ridges and covered by coelomic epithelium. The sperms and ova are derived from the primordial germ cells in the gonads.
The crypts or tubules lined with epithelium constitute the testes. The sperms produced in the tubules are motile and carried through vasa efferentia and vasa deferentia to the storage site (seminal receptacle) or copulatory organ or discharged to the exterior. The mesonephric ducts in male function as sperm ducts and retain their connections with the urinary system in the adult.
The connective tissue or stroma covered by epithelium constitute the ovaries. The primordial germ cells in the ovaries enlarge and form ova. In most cases, the mature ova are shed into the coelom, close to the infundibula or the funnels of the oviducts.
The oviducts carry the ova to the site of fertilization or to the exterior. The pronephric ducts function as oviducts in females and they are not connected with the urinary system in the adult.
In some fishes and amphibia the ova are shed into the coelom and pass out through the genital pore. In most vertebrates, the posterior part of the oviducts dilates to form uteri (ovisacs) or the two ducts may join and dilates to form a single, large uterus (human). In birds only the left oviduct persists in the adult.
Accessory reproductive organs:
Certain accessory structures are always associated with the gonads in all animals where the gametes are to be transported to the genital atrium or genital pore or to the uteri of females of the same species. The accessory structures and the glands in the walls of the ducts secrete fluid to facilitate movement of the gametes.
The oviducts and certain accessory glands play a major role in vitellogenesis, i.e. contributing yolk to the eggs and providing protective coverings, the vitelline membrane, the shell, leathery (in majority of the cases) or calcareous (reptiles and birds) and also in secreting a protective capsule enclosing eggs (insects, molluscs).
Spermathecae, small sacs for storing sperms after mating, are present in the females of many species. The sacs may or may not be connected with the oviducts.
Gonadal activities:
Animals do not breed round the year. The period in which a particular species breed is the breeding season for the species. It is related to geographical area, climate, food supply, etc. This varies in different animals. Broadly, animals thriving well in wet season, breeds in monsoon, while the majority of the others breed in the spring. The period may be stretched in either way.
The ovaries are active only in the breeding season. This is probably due to the limited number of primordial germ cells (4,00,000 in human) in the ovaries, destined to form ova. On the other hand, the testes are active throughout the year but not at equal tempo. The peak period of sperm production is the breeding season. This is presumably due to the capacity of an individual to produce an enormous number of sperms.
As a rule, the number of sperms produced by males is always much more than the eggs produced by the females of the same species.
This serves dual purpose, to compensate the death of a large number of sperms in course of their movement towards the ova, and though only a single spermatozoa is needed to fertilize an ovum, presence of a large number of them is required in the vicinity of the egg to produce the required amount of enzyme to dissolve the coverings of the egg.
In mammals, the enzyme, hyaluronidase secreted by the sperms dissolves the hyaluronic acid, the binding material holding the follicle cells around the ovum.
Gamete production:
A regulatory mechanism to control gamete production is present in all sexually reproducing animals. The gonads and the accessory structures are remarkably modified to meet environmental assaults.
Transmission from host to host to complete life cycle is compulsory in endoparasites, with a few rare exceptions. Large number of embryos perish in the process. To compensate the loss, the gonads have enlarged enormously in size to produce a very large number of gametes.
A female Ascaris, a round worm, produces 20,000 eggs a day, a single liver fluke, a flatworm, produces 50,000 eggs. In the tape worm, Taenia, a proglottid beyond 200 is a complete hermaphrodite unit. The proglottids from 400 onwards, i.e. the last 400 to 500 are fully occupied by a much branched uterus, heavily loaded with embryos.
Contrast to this, a drastic reduction in the number of ova produced is associated with internal fertilization and formation of a protective membrane, a leathery egg shell, or a calcareous egg shell or a similar structure around the egg after fertilization to protect the embryo.
In animals with intrauterine development, the security of the foetus is ensured and the number of ova produced at a time has come down to one to only a few. To provide yolk to the eggs is a tremendous physiological stress on the mother and the number of eggs produced is reduced with the enlargement of egg size. In birds, one oviduct has atrophied to prevent the development of many eggs at a time.
Fertilization:
The union of a sperm and an ovum is fertilization. Most sperms are motile but the distance they can travel in their brief life span in the open environment is rather small.
Fertilization takes place both outside and inside the female body and known as external and internal fertilization, respectively:
External fertilization:
External fertilization is possible only in animals living or breeding in water. Being attracted by chemical stimuli the motile sperms move to the passive ova and fertilize them. The union of the gametes is a matter of chance and many sperms die in their hazardous journey.
Factors in the surroundings also kill many ova and above all the life span of the gametes is very short. To ensure fertilization, millions of eggs are produced by a female in one spawn and still more number of sperms are released by the male, so that at least fertilization of a few eggs is ensured.
In addition, certain adaptations help in synchronizing the release of sperms and ova and that close to one another. Internal neural and hormonal controls are activated by both abiotic and biotic factors — temperature, photo-period and lunar and tidal cycles; courtship, stimulation received from shedding of gametes (sperms) of a member of the opposite sex at close proximity of the partner during ovulation period, are adaptations to synchronize gamete release.
Internal fertilization:
Fertilization of ova inside the body of a female is internal fertilization. This is advantageous because placing the sperms close to the ova greatly increases the chances of fertilization and thereby cut down the wasteful expenditure of living materials spent in the production of a large number of gametes.
At the same time it needs several adaptations, viz. space to accommodate the ova in the body of the female, at least for a certain period both in the pre- and post-fertilization phases. The eggs receive yolk and protective coverings, the shell and others after fertilization.
Internal fertilization occurs almost in all terrestrial animals. A fluid medium is necessary for the movement of the sperms towards the ova and a certain amount of moisture is required to prevent desiccation of the embryos during development. Both are lacking on land. The ducts through which the sperms move to reach the eggs contain fluid secreted by the walls of the duct or certain accessory glands.
The eggs laid by land animals contain sufficient amount of water required for the incubation period.
The loss of water through evaporation is prevented by an impervious covering and where such effective coverings are absent, eggs are laid in burrows in moist soil or covered by soil. Internal fertilization is not restricted only to higher vertebrates and terrestrial animals and occurs in many marine and fresh water invertebrates.