Wednesday, July 15, 2009

some things about sexual reproduction in animals

Sexual reproduction

Individual develops by process of gametogenesis, fusion, zygote formation and then embryogenesis.

Due to geneflow, variation is uniformly distributed in the population after a long time.

It maintains vitality of population

Let me introduce you to some terms

Protandry : male reproductive organs of an amphisexual organism matures first eg. Amphioxus, herdmania, sponges

Protogyny : female reproductive organs of a bisexual organism matures first eg. Earthworm

Seasonal breeder: breeds in only one season of the year. Eg. Birds usually breed in winter (except the birds in captivity)

Ant. Continuous breeder eg. Humans.

Sexual life of an organism can be broadly categorized as juvenile phase (pre-reproductive phase), mature phase (reproductive phase), and senescent phase (post- reproductive phase)

Hormones are responsible for changing of phases. Sex hormones inhibit transition to senescent phase.

Reproductive phase can be divided into menstrual, pre-menstrual and post menstrual phases in primates, and in non primate mammals, into estrous, pre-estrous and post-estrous phases.

In non primates, breeding season influences by hormones. Hormones are controlled by nervous system and photoperiodism.

Eg. Light regulates the reproductive cycle in many organisms.

Like in birds, breeding season is November to January because daylength is less during this time. Likewise, it is seen that rats cant reproduce if constantly kept in light.

Events of reproduction

Sexual reproduction can be divided into 3 phases.

Pre fertilization events : It involves gametogeneseis and gamete transfer.

Special methods of gamete transfer

  1. Gymnophiona ( icthyofish) : A member of Apoda, male has protrusible cloaca which enters into the female cloaca
  2. Necturus ( also mud puppy and salamander) : male release spermatophore and female picks it up thro cloacal lip.

In external fertilization, water is the medium of gamete transfer (because gametes will die if fluid medium is absent).

Fertilisation events : male gamete fuses with female gamete

Fertilization is of two types

  1. Endogamy = self fertilisatino
  2. exogamy = cross fertilisaitno

Exogamy is again of two types

  1. external fertilization
  2. internal fertilization

For external fertilisaitno, large no/- of gametes have to be produced in right areas and in right time for proper development and replacement of individual as chance of predation is high and life span of gametes is low.

Both gametes are released in same time (temporally) and in same place (spatially).

Post fertilization events : It includes zygogenesis and embryogenesis

Zygote : full of hybrid genome. It is vital link between 2 sucessive generations


  1. Oviparous (egg laying):eggs are either fertilized or unfertilised
  2. vivypary (born young)
  3. ovovivipary : fertilization and development in internal. The child is born young but it develops in egg inside female without taking nourishment from female


After fertilization, the whole sperm degenerates but egg is activated.

Sperm just activates the egg, but doesn’t contribute to genome.

Eg. Planaria, Durgesia


Development of individual from unfertilized egg is called parthenogenesis.

It is of two types

  1. Complete parthenogenesis: all individuals develop parthenogenetically. Hence, all individuals are females. Males are absent from in their species. Eg. Lacerta saxicola armeniaca (copasian lizard), Typhlina brahmina (smallest Indian snake)
  2. Incomplete parthenogenesis : Individuals develop both parthenogenetically and zygotically. Eg. Bee { queen bee = 2n, worker bee = 2n, Drone = n} only drones (males)are produced parthenogeneticaly.

Note : worker bee and queen bee(females) both are diploid and born from zygote. The queens egg laying apparatus changes to sting apparatus in worker bee. That’s why u can meddle with queen but not with her maids!!!

Aphid and Daphnia show cyclomorphosis.

In wasp, one generation, all are haploid (produced parthenogenetically) and next generation, all are diploid. This is called cyclic parthenogenesis

In turkey, partial parthenogenesis is seen. 40% males are parthenogenetic. 60% males and 100% females are produced by fertilized egg.

Parthenogenesis can also be divided into 3 as:-

  1. Arrhenotoky: only male develop parthenogenetically eg. Bees
  2. Thelytoky: only females develop parthenogenetically eg. Lacerta saxicola armeniaca, Typhlina brahmina.
  3. Amphitoky: both males and females develop by parthenogenesis eg. Aphids, Daphnia.

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