Evolution of sexual reproduction
By Dr Girish Chandra
SUMMARY
Asexual reproduction is generally quick, has less energy demands and is a simple process and hence the early protozoan adopted this method as the primary means of multiplication that enabled them to build up large populations in shortest possible time. Binary fission, multiple fission, budding, plasmotomy etc. are all asexual methods that are best suited to these animals which possessed delicate bodies and no apparent means of defence and therefore needed a speedy way of multiplication. Unlike in sexual reproduction, where only females reproduce, in asexual method all individuals produce offspring without the help of other individuals.
Asexual versus sexual reproduction
If asexual reproduction is simple and efficient, then why animals had to invent sexual method which is a complex and difficult process? Asexual reproduction apparently has two main disadvantages that it produces clones and does not produce diversity.
Sexual reproduction involves creation of two types of individuals producing two types of gametes whose primary aim is to exchange the genetic material between individuals and to bring about genetic diversity in the population. The power of recombination is so great that chances of two individuals being identical are almost nil. In the fiercely competitive world such a trait is advantageous and is favoured by natural selection.
Owing to overwhelming advantages in the struggle for life, an attempt to evolve sexual reproduction commenced in eukaryotes. A nucleus and the cell division apparatus were necessary to invent the process of meiosis that produced gametes carrying half the number of genes.
COST OF SEX
In sexually reproducing populations only 50% individuals produce offspring, while the rest half just contribute their genes, thus reducing the reproductive capacity, whereas in asexuals all individuals produce offsprings with no apparent help from others. Asexual reproduction saves energy and time.
Basic steps in the origin of sexual selection in Protozoa
First there was asexual reproduction in the beginning which was fast and simple but had its disadvantages as it produced clones and could not get rid of harmful effects of mutation.
· Different mating types originated.
· Anisogamy to some extent tried to solve the problem of cell fusion.
· Large number of mating types was reduced to two.
· Larger gametes specialized in the storage of nutrients and development of embryo.
· Larger gametes could now be produced in small numbers.
· Larger gametes being in small numbers became limiting resource, triggering competition.
· Individuals producing larger gametes were called females.
Theories of evolution of anisogamy
Isogamy produced equal sized gametes that had equal amount of energy investment. They were large cells, had low searching capacity and did not have particular attraction towards each other to fuse and form zygote.
Theory by Weismann et al.
Weismann (1866, 1902); Hertwig (1906); Butschli (1889) proposed that evolution of anisogamy from the primitive isogamy happened due to two selection forces:
- Due to increased efficiency in finding mating partner in anisogamy.
- To supply the zygote with sufficient amount of reserve food for development of embryo.
Origin of sexual dimorphism produced competition and increased the chances of fusion and also created a healthy sexual selection.
Theory of Kalmus (1932)
Kalmus (1932) and later Hoekstra (1984) gave the mathematical model for anisogamy.
EVOLUTION OF SEX IN PROTOZOA
Early protozoan probably reproduced by asexual means such as binary fission, multiple fission, budding etc. as these methods were quick, had lesser energy demands and all individuals participated in producing offspring. Asexual reproduction was best suited to animals which had delicate bodies and no apparent defence system and, therefore, must have needed a speedy way of multiplication for the recovery of population. Gametogenesis in protozoan is variable and can be classified in the following three forms:
- The protozoa in which adult (trophozoite stage) is haploid.
- Those protozoa in which adult stage is diploid.
- In ciliates gametic nuclei rather than gametes are formed.
- Haploid protozoan such as sporozoans, Elphidium etc., perhaps evolved when diploid individuals produced gametes.
- Paramecium adults produce gametes by meiosis and then by mitosis.
- Conjugation is a perfect sexual process in which exchange of genetic material takes place.
- In sporozoans and some other protozoans, haploid adults are formed and whole individual behaves like a gamete.
- In Elphidium, Volvox and mycetozoa, biflagellate isogametes are formed.
- Sexual reproduction rejuvenates protozoan populations instantly.