Facultative sex and reproductive strategies: are males always necessary?

by Maria Chiara Rosace, Giuseppe Scarlata, Antonino Napoleone | March 18, 2021.

Not all animals need to mate for reproduction, sometimes females are perfectly capable to reproduce without the help of a male. Are you surprised?
It is true, most of the time you need two essential ingredients, egg cell, and spermatozoon, to be able to procreate, but this is not always the rule. In biology, there is an unusual process, known as parthenogenesis, which allows some animals to reproduce without the help of the male. The offspring of parthenogenetically reproducing females can be thelytokous (i.e. made up of females only), arrhenotokous (i.e. made up of males only) or deuterotokous (in which there are offspring of both sexes). Most of the species that use this reproductive way are small invertebrates, such as bees, aphids, ants, or phasmids, which sometimes can alternate between sexual and asexual reproduction. Parthenogenesis has also been observed in approximately 80 species of vertebrates, mainly fish and reptiles!

How does parthenogenesis work?

Before talking about parthenogenesis, it is necessary to understand how sexual reproduction takes place.
Eukaryotic cells can divide in two ways: by mitosis and by meiosis. Mitosis is a process by which two daughter cells are formed from a single cell. This process generates two cells genetically identical to the mother cell and it occurs mainly during the cell division and renewal phases of all organisms, involving the somatic cells (any biological cell forming the body of an organism, with the exception of gametes). Meiosis, on the other hand, is an event that only involves gametes (Figure 1), the cells involved in reproduction. The gametes carry a complete set of genetic information, i.e. a copy of the genome of the organism from which they originate. The formation of new individual results from the union of two sex cells, each from one of the two parents.

So, if half the necessary genetic information is missing, how can individuals be formed by parthenogenesis? Parthenogenesis is a form of asexual reproduction in which an egg cell develops without being fertilised, giving rise to haploid or diploid offspring¹. During oogenesis (the process of formation of female gametes) the ovaries produce eggs through meiosis, which results in haploid cells (i.e. containing only one set of chromosomes). During meiosis, in addition to the oocyte, other haploid cells are also generated, known as polar cells. There is a form of parthenogenesis in which an egg cell merges with a polar cell, resulting in individuals similar to the mother, exclusively female, with a complete chromosome set, without the use of spermatozoa. In other cases, the basic processes of meiosis (reduction of the chromosome number from diploid to haploid) are completely or partially absent and from a genetic point of view, the descendants are once again identical to the mother. New genotypes may possibly arise only through mutations. In other cases, meiosis proceeds normally but the individual that develops parthenogenetically remains haploid.

Not all species use this mode of reproduction only. In most cases, parthenogenesis is alternated with amphigonic reproduction, i.e. sexual reproduction with fertilisation of the egg by the spermatozoon and formation of the diploid zygote.

Thelytokous parthenogenesis in Extatosoma tiaratum

Have you ever seen a stick insect? Actually, it is not so easy as it is particularly cryptic and active especially at night. The characteristic camouflage of these insects is a prime example of Darwin’s theory of evolution by natural selection. Various forms of camouflage have evolved in different species in response to predatory pressure. However, this is not the topic of today’s discussion. Stick insects are part of the order Phasmatodea (also known as Phasmida), which includes over 3000 different species! In Italy, the most representative one is Bacillus rossius, which is surprisingly twig-like. A common feature of many phasmid species, including our B. rossius, is the reproduction by parthenogenesis.

As already mentioned, some species can reproduce either by amphigonic reproduction or by parthenogenesis. But what determines the choice of one reproductive strategy rather than another? What drives females to lay eggs in the absence of a male? An interesting explanation comes from a study published in the Australian Journal of Zoology² a few years ago. The protagonist is a species of Australian phasmid known as Extatosoma tiaratum, the spiny leaf insect (video below). Both females (~12 cm in length) and males (~8 cm in length) of this species are known to occur in the wild, and they can reproduce either by copulation or asexually. The duration of copulation is usually very long, as with most phasmids, and can last up to 15 hours.

Females adjust their asexual ovipositing rate in response to their exposure to males during their growth. The presence of males in the early stages of development can give an indication of the actual likelihood of mating, so virgin females that grow up in the presence of males initially tend to lay fewer eggs by parthenogenesis in view of prospective mating at some later stage. Conversely, females that fail to meet males during their immature stages are less likely to meet a male in the wild even once they have reached sexual maturity. This leads to greater production of parthenogenetic eggs. Natural selection, therefore, favours facultative thelytoky in environments in which mating is difficult or impossible³.

Why do we need males?

From an evolutionary point of view, sexual reproduction might sound like a paradox: if it is possible to reproduce asexually, without a male’s help, why is parthenogenesis only widespread in a small portion of the animal kingdom? Parthenogenesis can have numerous advantages in stable environments or in environments in which mating is difficult or impossible, as we mentioned earlier talking about E. tiaratum.
However, sexual reproduction remains evolutionarily more advantageous because it ensures genetic variability, giving individuals greater plasticity in terms of adaptation to environmental changes. Genetically identical individuals are more vulnerable to disease and change than heterogeneous groups⁴.

“It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change.”

The author of this quote, based on Darwin’s theory, is uncertain.

References

1. Molnar C, Gair J. Animal Reproduction and Development: Reproduction Methods. In: Concepts of Biology. BCcampus; 2015.
2. Schneider A, Elgar MA. Facultative sex and reproductive strategies in response to male availability in the spiny stick insect, Extatosoma tiaratum. 2010. doi:10.1071/ZO10012
3. Brock P, Hasenpusch J. The Complete Field Guide to Stick and Leaf Insects of Australia. CSIRO Publishing; 2009. Accessed March 2, 2021. https://www.publish.csiro.au/book/6012/
4. Reece JB, Urry LA, Cain ML, Wasserman ML, Minorsky P V., Jackson RB. La Forma e La Funzione Negli Animali. Vol 5. 1st ed. (Brizzi R, Stefani M, Taddei N, eds.).; 2009. Accessed March 12, 2021. https://www.pearson.it/opera/pearson/0-6063-campbell_la_forma_e_la_funzione_negli_animali?iesError=login_required&iesErrorDescription=Login+required