REPRODUCTION. – Reproduction, or the faculty of generating new organisms similar to oneself, is the most typical and fundamental characteristic of living beings. It ensures the perpetuation of the species despite the mortality of individual organisms.
I. ANIMAL REPRODUCTION
In animals, various modes of reproduction are known, which can nonetheless be grouped into two main categories that differ fundamentally from one another: asexual (or agamic) reproduction and sexual reproduction. The former, less widely distributed but frequent especially among protozoa and some groups of invertebrates, essentially consists in the separation of one or more parts of the body from a preexisting individual, which then complete their development and transform into as many offspring. The latter, vastly more widespread, consists instead in the formation of a new organism from an initial cell (zygote), derived from the fusion of two special cells (gametes), one male (spermatozoon) and the other female (ovum).An essential difference between the two types of reproductive activity lies in the fact that animals born through asexual reproduction retain a hereditary patrimony identical to that of the parent, whereas, by contrast, organisms arising from sexual reproduction possess a hereditary patrimony that is a blend of traits derived from both father and mother. This has fundamental importance for the phenomena of individual variability and for the laws governing the transmission of traits.
1. Asexual reproduction
The phenomena of asexual reproduction occur in many different ways across various animal groups. In protozoa, the single cell constituting the entire organism frequently reproduces by simple fission, dividing into two identical cells that detach and grow to their original size. At other times, multiple fission occurs, in which the cell nucleus divides repeatedly, giving rise to many daughter nuclei, each of which becomes surrounded by a small amount of cytoplasm, thus generating as many independent individuals; generally, the process is preceded by the formation of a common protective envelope (cyst), which ruptures at the end of the reproductive activity. A third mode is budding, or the formation of one or more small cellular protuberances, each containing a nucleus derived from the division of the preexisting nucleus; each bud thus formed detaches from its parent and then grows, leading an independent life. Similar phenomena of simple or multiple fission and budding are also found in many metazoans, especially in coelenterates, flatworms, annelids, and tunicates. A well-known case is that of the freshwater hydra (Hydra sp.), which, in addition to reproducing sexually, is capable of proliferating on the sides of its body through budding one or more offspring destined to detach. In other coelenterates, through a similar process of asexual reproduction, entire colonies of polyps arise from an individual born through sexual reproduction; these polyps sometimes exhibit morphological differences linked to a complex division of labor for the sake of the collective life of the colony (e.g., siphonophores). Among flatworms, some strains of planarians have almost entirely lost the ability to reproduce sexually and multiply almost exclusively through fission (Benazzi). Among annelids, cases are known of polychaetes and oligochaetes forming entire chains (stolons) of organisms destined to detach; sometimes, in polychaetes, only these individuals produced by fission (schizosocoids) are capable of maturing the eggs and spermatozoa necessary for sexual reproduction (schizogamy, Malaquin).In various animal groups, there is a regular alternation between generations reproducing asexually and those reproducing sexually. The phenomenon, called metagenesis, has already been mentioned in reference to those elementary organisms in which colonies of multiplying individuals arise from an initial individual derived from a fertilized egg. In these cases, the animals of the generation producing eggs and spermatozoa typically have the appearance of medusae and derive through budding from those of the sexual generation, which have the form of polyps. Metagenesis was discovered by Chamisso (1818) in certain tunicates (salps), which can exist either as solitary individuals (solitary salps) or as chains composed of several individuals joined together (aggregate salps). Solitary salps arise sexually through fertilization between eggs and spermatozoa produced by aggregate salps; the chains of the latter, in turn, form asexually from a proligeroid stolon generated by the solitary salp, which is incapable of maturing male and female gametes.
A particular form of asexual reproduction is polyembryony, or the formation of two or more twin embryos from a single fertilized egg, which divides during early stages of ontogeny into two or more distinct parts that develop independently. The phenomenon, occasional in the human species, is habitual in certain insects and in some mammals (e.g., the armadillo).
2. Sexual reproduction
The first indispensable act of sexual reproduction is the preparation of the gametes. In protozoa, the single cell constituting the entire organism in certain phases of the life cycle can acquire the value of a gamete. In metazoans, composed of multiple cells, the gametes instead derive from a particular line of elements (germ cells) that, in the gonads (ovaries, testes), undergo a complex maturation process, transforming into eggs or spermatozoa. The most important phenomenon in gamete preparation is the reduction by half of the number of chromosomes characteristic of each species of organism. It is customary to say that gametes possess a haploid number of chromosomes, in contrast to other cells, which possess a diploid number. The meeting and fusion of two gametes of opposite sex to form the zygote is called fertilization. This can be external, as in numerous aquatic animals (sea urchins, annelids, fish, anuran amphibians, etc.), which release their gametes into the surrounding environment, or internal, when, by contrast, the male gametes are introduced into the female genital tract, where they encounter the egg. The spermatozoon, small and mobile due to its possession of a tail-like flagellum, reaches the egg and penetrates it; the surface of the egg then changes with the appearance of a fertilization membrane, which prevents the entry of other spermatozoa. The two haploid nuclei of the egg and the spermatozoon (called the female and male pronuclei) then fuse together, forming a diploid nucleus (the nucleus of the zygote); and a process of mitosis in this nucleus marks the beginning of the egg’s development.
From a physiological standpoint, one may consider that the mature unfertilized egg is a cell in a state of dormancy that the entry of the spermatozoon awakens to new life, as is frequently shown by the resumption of respiratory activity and exchanges of saline substances with the environment.
A form of sexual reproduction, called parthenogenesis or virgin birth, is that in which the egg is capable of generating an organism even in the absence of the activating action of the spermatozoon. The phenomenon, discovered in certain insects (aphids) by C. Bonnet in the 18th century, is now known, as a normal means of reproduction, in many other invertebrates as well. Thus, in the bee and in the generality of hymenopterans, females and males are born depending on whether the egg is or is not fertilized, whereas in many other cases (phylloxera, cladocera, rotifers) there is a more or less regular alternation between one or more parthenogenetic generations and a generation born from a fertilized egg (heterogony).
With various experimental means, whether physical or chemical, eggs that in nature can develop only if fertilized may be induced to undergo parthenogenesis. Thus, in the egg of the sea urchin, the activating effect of the spermatozoon can be replaced by various chemical actions (Loeb), and in the eggs of the frog the same effect is obtained by pricking the virgin egg with a fine needle dipped in blood (Bataillon). Recently, even in the rabbit, experimental parthenogenetic development of the eggs has been achieved (Pincus, Olivo).
3. Oviparous and viviparous animals
Animals that lay their eggs in the external environment at a very early stage, when embryonic development has only just begun or even before it has started, are called oviparous. These eggs are usually enclosed in a gelatinous envelope if they develop in water, while they are protected by resistant shells when deposited in a more or less dry environment. Among oviparous animals, some show no care for the eggs they have laid or for the young that hatch from them, whereas others lavish a series of attentions on the eggs and the newborn, generally known as parental care. These attentions may consist in the construction of a nest, of the most varied shapes in different animal groups, designed to provide a safe shelter for the eggs and offspring; or they may involve keeping the offspring close to the parent; or even offering asylum on the parent’s own body. Thus, the females of the anuran amphibian Pipa americana deposit their eggs on their own backs, around each of which the skin forms a small cell, and within this the embryo develops. In the sea horse (Hyppocampus), it is instead the male that carries the eggs laid by the female, incubating them within a special abdominal cutaneous pouch. In addition to many invertebrates, oviparous animals include the majority of fish and amphibians, many reptiles, all birds, and among mammals, only the monotremes (echidna, platypus).In contrast to oviparous animals, those whose eggs are not laid early in the external environment but remain for a variable time in a special cavity of the maternal genital apparatus, where embryonic development takes place entirely or for the most part, are called viviparous.
In many cases, the embryo, throughout the time it is housed within the mother, does not derive its nourishment from her but instead draws exclusively on reserve substances that were stored in the egg from the beginning in the form of yolk globules. In such cases, one speaks more precisely of ovoviviparous forms, examples of which are found, in addition to invertebrates, among teleosts (gambusia), selachians, amphibians (Salamandra atra), and reptiles (vipers). Among mammals, marsupials may perhaps be considered ovoviviparous, since the embryos do not establish close relations with the maternal organism and are born at a very early stage. Sometimes, however, the embryos soon form close connections with the walls of the maternal cavity (uterus) in which they are contained and are nourished for a long time at the mother’s expense by means of a special organ (placenta) that connects to her. In such cases, one speaks of true viviparity, which is found in some invertebrates, in a few selachians, in a few reptiles, and in the vast majority of mammals (the so-called placental mammals).
Even ovoviviparous and viviparous forms often exhibit parental care toward their offspring after birth. It will suffice to recall the function of suckling, characteristic of all mammals and especially prolonged in forms with helpless young.
II. PLANT REPRODUCTION
Reproduction in the plant kingdom is of such importance that it serves as the basis for the systematic classification of a large number of plant groups (classes, subclasses, orders, suborders). Indeed, Linnaeus founded his “sexual system” on the basis of sexual reproduction, and it still holds sway today.In plants, reproduction can be sexual or asexual. Sexual or gametic reproduction is more complex than asexual or agamic reproduction, which involves vegetative multiplication. The latter can involve various parts of the same individual and can replace sexual reproduction when the latter is lacking for any reason. A fundamental characteristic of sexual reproduction is the formation and subsequent union of two gametes that, through a sexual act (amphimixis), contribute to the formation of the zygote, which represents the first cell of a new organism derived from two parents or from two cells that have united for this purpose. In plants, asexual reproduction is already more or less uniform and provides for the reproduction of existing parts, which, by simple detachment from the producing individual, give rise to a new individual that is in no way constitutionally different from the one that produced II. In contrast, sexual reproduction tends to generate individuals that vary in their chromosomal makeup within the range of possibilities of genomic variation.
To gain a complete picture of the sexuality of botanical species, it is well to review the variations among the lower and higher groups of the botanical scale. The lowest groups (bacteria or schizomycetes) lack
Sexuality either does not exist or is not known. There exists only the so-called “sporulation,” i.e., the formation of endogenous spores in a cell which, upon rupture of the membrane, are released. In micromycetes there are equal gametes (isogametes, isogamy). In algae sexuality is very evident, particularly because a sexual generation alternates with an asexual generation; these two phases conclude with the formation of the zygote, i.e., the union of two haploid cells (gametes) into a single diploid cell (zygote), and with the reductional division (meiosis) that restores the haploid chromosome complement in the sexual cells. In some groups the two processes are separated in the sense that a soma (haploid or diploid) is interposed between them, but sometimes, as soon as the zygote is formed, the first division it undergoes may be the reductional one, so that the only representative of the diploid phase is the zygote itself. Such groups are called haplobionts. If, on the other hand, as in the case of higher plants and animals, the reductional division immediately precedes the formation of the gametes and thus that of the zygote, a diplobiont is present, inasmuch as the diploid phase has absolute predominance over the haploid, which is represented only by meiosis. In the intermediate case, when in the life cycle of an organism the haploid phase has a long series of divisions, at the end of which gametes are formed, and likewise the zygote undergoes a long series of divisions giving rise to a soma, it is said to be an haplodiplobiont.
In general, green algae are haplobionts, in which the gametophytic phase, bearing gametes, predominates, although the asexual reproduction by zoospores (free cells that vegetatively reproduce the algal filaments) is not absent (e.g., Ulothrix), and the sporophytic phase is reduced to the zygote alone, which undergoes meiosis (reductional division) as soon as it is formed. Some brown algae are haplodiplobionts (e.g., Laminaria), in which the sporophytic phase predominates and the gametophytic phase is reduced to a few divisions; others (e.g., Fucus) are haplobionts because the sporophytic phase predominates over the gametophytic phase, which is reduced to the two reductional divisions. Red algae are also haplodiplobionts because the gametophytic phase extends beyond the two reductional divisions and the sporophyte is represented by the sporogenous filaments. Asexual spore reproduction (zoospores) is very common in algae and, alongside sexual reproduction, represents a purely vegetative form of multiplication.
The alternation of generations in fungi is centered on the production of spores that give rise to haploid thalli. The zygote in some cases (lower Ascomycetes) is the sole representative of the sporophytic generation. In higher Ascomycetes, the mingling of the cytoplasm (plasmogamy) precedes the fusion of the two nuclei (karyogamy). The ascogenous hypha with its dikaryon is considered part of the sporophyte.
Basidiomycetes show the two phases in equilibrium and are therefore considered haplodiplobionts. The reproduction of lichens, biological forms resulting from a mutualistic parasitism between green algae and fungi, is mostly vegetative, and sexuality, as well as classification (Ascolichens and Basidiolichens), is entrusted to the fungus, which exhibits the same reproductive organs as fungi.
Bryophytes entrust reproduction and multiplication to organs as well as to vegetative parts. In their life cycle the gametophytic phase predominates. A spore produces the protonema on which a plantlet arises, bearing at its vegetative apex the archegonia (female organs bearing the egg) and the antheridia (male organs bearing antherozoids). The motile antherozoid fertilizes the immobile egg, which, in place, forms the zygote that produces the sporophyte. The soma is therefore represented by the gametophyte.
In Pteridophytes the sporophyte predominates over the gametophyte, which is reduced to a prothallus. The fertilized egg develops in place (zygote) and produces the sporophyte, which represents the soma. In higher ferns, the prothalli, undifferentiated in lower ferns, bear only archegonia (female prothalli) or only antheridia (male prothalli) that produce, respectively, megaspores and microspores. Such ferns are said to be heterosporous.
Phanerogams still exhibit an alternation of generations, but the gametophytic phase is borne by the sporophyte and is confined within the ovule, which in Gymnosperms is borne by the open carpel and in Angiosperms by the closed carpel or ovary. In Gymnosperms the spore derived by meiosis from a cell of the nucellus produces a prothallus (endosperm) on which a rudimentary archegonium bears the egg, which is fertilized by the sperm nucleus carried by the pollen (mobile male gametophyte). The zygote produces the embryo, which develops in place and at the expense of the endosperm, transforming the ovule into a seed.
In the nucellus of the ovule of Angiosperms, out of a tetrad of megaspores three degenerate and only one megaspore germinates, giving rise to the gametophyte that bears the egg. At the same time, the pollen produced in the stamens carries the sperm nucleus that fertilizes the egg. The flower is the seat of the union, but the flower may have the two sexes united (hermaphrodite or monoclinous flower), or two distinct flowers bear, one the gynoecium and the other the androecium (monoecious plant), or two distinct individuals bear separate male and female flowers (dioecious species). Dioecism and monoecism are forms that do not disturb sexuality at all, which is favored by external factors such as wind, animals, water, etc. (anemophilous, entomophilous, hydrophilous pollination, etc.). Nature seeks to avoid self-fertilization, i.e., fertilization of the ovule by pollen from the same flower.
Besides sexual reproduction, there is also vegetative multiplication, whereby a part of the soma, detaching from an individual, generates another individual entirely similar to the producer. Thus, an olive branch can regenerate a plant entirely similar to the one that produced II.
In botanical species, besides forms of sexual reproduction and vegetative multiplication, there are other forms: parthenogenesis, which consists in the development of the egg in the absence of the male gamete; apospory (suppression of spore formation), in the case where a diploid cell of the nucellus develops and, invading the space destined for the gametophyte (which aborts), gives rise to a diploid embryo sac that degenerates; apogamy, i.e., the development of a cell of the gametophyte other than the egg and therefore haploid, etc.
Sexual reproduction allows for the exchange of genes and thus variability within the limits of the species among individuals belonging to it; it also allows for the formation of interspecific and sometimes even intergeneric hybrids, these mostly infertile. In contrast, vegetative multiplication allows only for the vegetative development of the soma.
BIBL.:
Martinoli, G., *La riproduzione sessuale e vegetativa nelle piante*, Roma 1948.