The ability for females to reproduce without males, known as parthenogenesis, is quite rare among vertebrates but does occur in some species. Here we’ll explore what parthenogenesis is, look at examples of vertebrate species that can reproduce this way, discuss the evolutionary advantages and disadvantages of parthenogenesis, and examine if it could ever occur in humans.
What is Parthenogenesis?
Parthenogenesis is a form of asexual reproduction where an unfertilized egg develops into a new individual. This means females can reproduce without mating with a male. The offspring produced from parthenogenesis are clones of the mother and inherit all her genetic material.
Parthenogenesis occurs naturally in a variety of invertebrate species including insects, crustaceans, nematodes, and rotifers. It’s much rarer in vertebrates but has been observed in some reptiles, amphibians, fish, and birds.
How Parthenogenesis Works
In sexual reproduction, offspring inherit half their DNA from each parent resulting in genetic diversity. In parthenogenesis the offspring are genetically identical clones of the mother.
This occurs because the mother’s eggs undergo abnormal cell division. Instead of an egg fusing with a sperm during fertilization, the egg cell divides on its own through mitosis. The offspring develops from that single cell and contains only the mother’s DNA.
Examples of Vertebrate Parthenogenesis
While uncommon, several vertebrate species have exhibited the ability to reproduce through parthenogenesis including:
Reptiles
- Lizards – several whiptail lizard species
- Snakes – Burmese pythons, copperheads, boa constrictors
- Monitor lizards
- Geckos
Amphibians
- Salamanders
- Frogs – Cremea toad
Fish
- Hammerhead sharks
- Blacktip sharks
- Bonnethead sharks
- Zebra sharks
- Bonefish
- Gynogenetic fish species
Birds
- Turkeys
- Chickens
- Pigeons
- Ducks
In reptiles, parthenogenesis has been most studied in whiptail lizards. In several whiptail species, females can reproduce through parthenogenesis and populations consist entirely of female clones. All individuals are genetically identical and males do not exist.
In birds like chickens and turkeys, parthenogenesis can occasionally occur but the offspring do not survive or develop completely. However, the ability is present in females of these species.
Evolutionary Advantages and Disadvantages of Parthenogenesis
The ability to reproduce asexually without males provides some advantages but also has drawbacks in terms of evolution and genetic diversity:
Advantages
- Can establish populations rapidly from a single individual
- Useful when males are scarce or finding a mate is difficult
- Pass on genes directly without modifications from mating
- Avoid costs associated with mating
Disadvantages
- Offspring are genetically identical clones, low genetic diversity
- Harmful mutations accumulate over successive generations
- Makes adaption to changes in the environment slower
- Higher susceptibility to disease
The advantages allow species to colonize new areas quickly and reproduce successfully when mating opportunities are low. However, over time the disadvantages of low diversity become problematic and can threaten the survival of parthenogenetic populations.
Is Parthenogenesis Possible in Humans?
Parthenogenesis has never been observed in mammals, which depend heavily on genetic recombination during reproduction. However, in theory it may be possible through technologies like cloning. Here are some key considerations about parthenogenesis in humans:
Natural Parthenogenesis Not Possible
Spontaneous parthenogenesis does not occur naturally in humans or other mammals. The cell division process required likely requires genetic elements not present in mammals. Egg cells initiate division after being activated by sperm cells during fertilization. Without sperm, human egg cells cannot divide or develop normally on their own.
Parthenote Embryos Not Viable
Even if a human egg began dividing parthenogenetically, the embryo would not implant or survive because it requires paternal genes. Research has produced parthenote embryos (from an unfertilized egg) using cloning methods in primates like monkeys. However, these parthenote embryos did not develop properly and died quickly.
Reproductive Cloning Theoretically Possible
While natural parthenogenesis isn’t possible for humans, reproducing without males could theoretically be achieved through reproductive cloning technology. This involves transferring the nucleus of an adult human cell into an enucleated egg cell which can then divide and develop into a clone of the original person. Here is a summary:
| Reproductive Cloning Process |
|---|
| 1. Obtain cell with DNA from person to be cloned |
| 2. Extract nucleus containing DNA from cell |
| 3. Remove nucleus from egg cell |
| 4. Transfer nucleus into enucleated egg cell |
| 5. Stimulate cell divisions artificially |
| 6. Implant cloned embryo into a surrogate mother |
| 7. Surrogate gives birth to clone with identical DNA |
Reproductive cloning could theoretically allow females to reproduce without males. However, there are still no examples of successful human cloning and major technical limitations remain before it could become feasible.
Conclusion
Parthenogenesis offers interesting examples of alternative reproductive strategies in some vertebrates but is not naturally possible for humans or other mammals. While females can reproduce asexually in some reptiles, amphibians, fish and rarely in birds, mammals require genetic material from males and females. Technologies like cloning may offer possibilities for human parthenogenesis someday but significant barriers remain. Overall, mammalian reproduction will continue to involve genetic contributions from both males and females into the foreseeable future.