Alihan Katlav: Egg size matters for sex allocation in thrips

Alihan Katlav is currently a Ph.D. candidate at the Hawkesbury Institute for the Environment, Western Sydney University. Alihan is studying the evolution of sex allocation in haplodiploid thrips. Under supervision of Assoc. Prof. Markus Riegler and Prof. James Cook, Alihan’s Ph.D. research focuses on the mechanisms and constraints of sex allocation adjustment in Kelly’s citrus thrips – an important Australian-native pest of citrus which is also invasive in New Zealand, other Pacific Islands, and Mediterranean countries. In this insight, Alihan presents his recently published research article “Egg size-mediated sex allocation and mating-regulated reproductive investment in a haplodiploid thrips species” in Functional Ecology.

Author Alihan Katlav, Hawkesbury Institute for the Environment Western Sydney University, NSW, Australia
Author Alihan Katlav, Hawkesbury Institute for the Environment Western Sydney University, NSW, Australia

About the paper

A key aspect of reproduction in all animal species is the decision about the allocation of resources to male versus female offspring. This generally involves the adjustment of offspring sex ratio, with a diversity of underlying mechanisms among different animal taxa. Of particular interest are haplodiploid arthropods with diploid females and haploid males; females develop from fertilized eggs while males develop from non-fertilized eggs. It has widely been recognised that females of haplodiploid species can have an adaptive control over fertilization (and therefore over sex allocation) through regulating the sperm access to their eggs after mating. This is well demonstrated in hymenopterans, such as in social bees with queens that lay female-destined eggs in larger cells (therefore better provisioned cells) and male-destined eggs in smaller cells. However, the underlying mechanism whereby a female can adjust the offspring sex in response to the environmental cues was unknown in haplodiploid insects.

For the first time in haplodiploid insects we deciphered a mechanism of sex allocation adjustment. Using the haplodiploid thrips species Kelly’s citrus thrips, Pezothrips kellyanus as a study species, we detected an egg size-dependent fertilization mechanism. This study was inspired by previous research on a spider mite species by Macke et al. (2011) that showed larger eggs were more likely to be fertilized. However, the lack of sufficient information regarding the fertilization system in mites left the exact mechanism by which egg size mediates fertilization unclear. Our study provided empirical evidence that sperm access to eggs is linked to direct mechanical stimulation of spermathecal muscles depending on egg size. We found that there is a greater chance that larger eggs become fertilized, likely by stimulating the spermathecal muscles in a female’s reproductive tract resulting in delivery of sperm stored in the spermathecal to the egg. Furthermore, our study showed that mating can induce mothers to produce larger eggs and thereby more female offspring. This mating effect may reflect natural selection on males that favours the production of more daughters over sons: in haplodiploid species males can only pass on their genes through production of female offspring. Overall, our study showed the ability of egg size, and male ability to manipulate it, to influence sex allocation patterns in haplodiploid insects.

About the research

Disentangling the mechanisms of sex and resource allocation, sex ratio adjustment patterns and their effects on life history traits of parents and offspring can provide insights into understanding the interplay of evolutionary and ecological dynamics of insects. Such information can subsequently be used for the development and application of pest management strategies. The findings of this research may further prompt the study whether and how maternally inherited endosymbionts can manipulate sex allocation in their host in favour of more female production to boost their invasion rate in the host population, and therefore substantially contribute to the population dynamics of endosymbionts in haplodiploid hosts.

About the author

Alihan (right) and his mother, Shams Paryav (left) with a view of blue mountains, NSW in the background, portrayed by a close friend of Alihan, Shima Arian Nezhad in 6th Jun 2020.
Alihan (right) and his mother, Shams Paryav (left) with a view of blue mountains, NSW in the background, portrayed by a close friend of Alihan, Shima Arian Nezhad in 6th Jun 2020.

I am an entomologist and throughout my life I have always been fascinated by the remarkable diversity of arthropods (especially insects), and this is mostly attributed to the inspiring role of my mum who, as a genuine nature lover, has spawned this passion in me since childhood. The way animals have evolved has always been puzzling for me and this has led my Ph.D. research towards the study of sexual selection as a driving force in evolution. My project currently investigates the role of bacterial endosymbionts in the reproductive biology of thrips with the focus on the impact of Cardinium and Wolbachia on the evolution of sex allocation and sexual conflicts in their hosts. Beyond this, I am an enthusiastic taxonomist and one of my research interests is the identification and study of the biodiversity of parasitic mites associated with insects, and their role in the ecology and population dynamics of their insect hosts. So far, I have described 12 new species of mites and I am currently one of the subject editors of the journal Zootaxa.

Read the article in full here.

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