Clownfish in a fish tank

Unconventional reproduction

Offspring aren’t necessarily produced by one male and one female. Leigh Ansell looks at self-pollination, hermaphroditism and the science behind ‘virgin births’

Asexual reproduction

Asexual reproduction comes in many different forms. It doesn’t always rely on a single cell dividing in two – though this is the method of choice for some organisms, like amoeba and bacteria. Certain types of worm, for example, split their body into parts in a process known as fragmentation, with each part able to grow into a whole new organism. Yeast goes for a slightly different tactic, called budding, which involves small outgrowths on its cell surface that detach when fully mature.

But it doesn’t end there. Some animals go for a more unusual approach – another type of asexual reproduction called parthenogenesis.

Roughly translating from Greek as ‘of virgin origin’, this process allows an embryo to develop without fertilisation. It means females of a species can reproduce using their own eggs, without needing a male.

Komodo dragons are perhaps the best-known example. These lizards are capable of both sexual and asexual reproduction – and which one they choose depends on the availability of a mate. With males around, they’ll reproduce sexually. But if there aren’t any, the female can produce offspring on her own. It’s a useful adaptation that allows species to survive in difficult environments – for example, if population numbers are small or they’re kept isolated in zoos.

Mammals aren’t capable of parthenogenesis. This is because their chromosomes are labelled according to whether they come from the mother or father, and both are needed for an embryo to develop normally. Only one parthenogenetic mammal has ever lived – a mouse created by Japanese scientists in 2004. This was only made possible through genetic modification and would never happen naturally.

Reproduction in plants

Even sexual reproduction doesn’t always involve two organisms. Some plants rely on autogamy, which means they self-pollinate. Pollen from the anther (the male part) is transferred to the stigma (the female part) of the same flower. This process is seen in a number of plants, including wheat, tomatoes and chilli peppers. It has several advantages – for one thing, seed production is guaranteed, and the plant can get away with producing less pollen overall.

For others, however, self-pollination isn’t an option. Corn, carrot and onion all require allogamy – in other words, they need to be cross-pollinated by another plant. Though it’s not quite as simple as self-pollination, its main advantage is the genetic diversity it produces, which allows the population to adapt to changes in the environment. So how do these plants avoid being pollinated by themselves?

The answer lies in a feature called self-incompatibility. Plants have a number of different ways through which they can block self-pollination. Their flowers may have male or female parts only – or if they have both, they may mature at different times. Alternatively, members of the plant population could be one of two structural types, each preferring to be pollinated by the other. However, overall, genetic and biochemical mechanisms form the most effective natural barriers against autogamy.

Unusual sexual reproduction in animals

But that’s plants – surely sexual reproduction in the animal kingdom is more straightforward?

Not quite. Some species demonstrate hermaphroditism – that is, they have both male and female sexual organs. For some animals, like snails, these are present and functional at the same time, so they’re called simultaneous hermaphrodites. Without defined sexes, all organisms can produce offspring, giving them a clear advantage over species where only half the individuals can bear young.

Clownfish, on the other hand, are sequential hermaphrodites. They’re all born male and live together in groups, with the most dominant fish in a group turning female later in life. It’s thought this is useful because clownfish tend to stick close to their sea anemone homes, limiting their potential to find a mate. By adapting to the surrounding population and making sure a female is always present, breeding becomes much easier. 

So perhaps there’s no such thing as conventional reproduction. Some of these methods might seem odd, but these adaptations serve a purpose – they’re part of the bigger picture of helping these species survive in their environment.

Lead image:

Clay vanSchalkwijk/Flickr CC BY

References

Questions for discussion

  • Parthenogenesis is more common in some climates than others. Why might this be?
  • Why does parthenogenesis affect the gender ratio of a population?

Further reading

About this resource

This resource was first published in ‘Sex and Gender’ in December 2016.

Topics:
Genetics and genomics, Ecology and environment
Issue:
Sex and Gender
Education levels:
16–19, Continuing professional development