Parthenogenesis is a reproductive strategy where a female animal produces offspring without the genetic contribution of a male. The term is derived from the Greek words parthenos (virgin) and genesis (creation). This form of self-reproduction allows an embryo to develop from an unfertilized egg cell. While sexual reproduction is the prevailing method, parthenogenesis is common among invertebrates, offering an efficient way to rapidly increase populations. Its occasional appearance in more complex animals has fascinated biologists.
The Science of Parthenogenesis
The challenge in parthenogenesis is transforming a haploid egg cell, which contains half the necessary genetic material, into a viable, diploid embryo. Normally, meiosis halves the chromosome number and produces polar bodies; sperm fertilization restores the full set. In self-reproduction, the female uses cellular mechanisms to bypass sperm and restore the diploid state.
One common method is automixis, where the haploid egg fuses with a polar body, restoring the chromosome count. Because genetic recombination occurs during meiosis, the offspring are semi-clones, not perfect copies. A different mechanism, apomixis, suppresses meiosis entirely. The egg cell develops directly via mitosis, resulting in an offspring that is a near-perfect genetic copy of the mother.
Invertebrates That Rely on Self-Reproduction
For many invertebrate species, parthenogenesis is the primary reproductive strategy, known as obligate parthenogenesis. This method allows a single individual to establish a population quickly without expending energy to find a mate. Bdelloid rotifers are a remarkable example, as no male has been observed in their lineage, suggesting they have reproduced asexually for millions of years.
Insects like aphids exhibit cyclic parthenogenesis, alternating between asexual and sexual reproduction based on environmental conditions. During favorable seasons, they clone themselves rapidly to maximize population growth.
The order Hymenoptera (bees, wasps, and ants) uses a system called haplodiploidy, where the sex of the offspring is determined by fertilization. Female offspring (workers and queens) are diploid and develop from fertilized eggs. Male offspring (drones) are haploid and are produced parthenogenetically from unfertilized eggs, making male production a straightforward process of self-reproduction for the queen.
Facultative Parthenogenesis in Vertebrates
Parthenogenesis is surprising when it appears in vertebrates, which are overwhelmingly sexual reproducers. These cases are known as facultative parthenogenesis, where the female switches to asexual reproduction when a male is unavailable. This phenomenon has been documented across fish, reptiles, and birds, and is considered a last-resort reproductive strategy.
The most publicized examples involve reptiles, such as the Komodo dragon and several species of snakes, including boa constrictors and pit vipers. For these animals, self-reproduction often relies on the automixis mechanism. Because sex is determined by the ZW system in many reptiles and birds (females are ZW, males are ZZ), this process can result in the production of exclusively male (ZZ) offspring.
This type of self-reproduction has also been observed in several shark species, including the bonnethead and the zebra shark, typically in females isolated in captivity. The ability to produce offspring without a mate provides a mechanism for species survival when females are geographically isolated or mates are scarce. Rare, isolated cases have even been recorded in birds, such as domestic turkeys and chickens.