Evolution of genetic systems & sex chromosomes

Genetic system - Lysiphlebus
Lysiphlebus fabarum, a parasitoid wasp with parthenogenetic females (photo: Christoph Vorburger)

Animals exhibit a staggering diversity in how genes are transmitted from one generation to the next. Many engage in normal sexual reproduction with two different sexes (e.g., our own species) or with hermaphrodites (e.g., many snails). However, many other species – for example ants, bees, wasps and many mites – have genetic systems where males are produced asexually and are haploid, whereas females are produced sexually and are diploid (haplodiploidy). Finally, a few animal species have even abandoned sexual reproduction altogether and consist of asexually reproducing females only (parthenogenesis).

There are also many ways in how females and males are determined in animals with two distinct sexes. This may be achieved through environmental cues like temperature, or genetically with one or more sex determining genes. Genetic sex determination also involves a wide range of different sex chromosome systems, from the familiar XY to the opposite ZW system in birds and butterflies and more exotic systems like the XXXXXYYYYY system found in the platypus.

Genetic system - X inactivation
Genotypes and their fitness in a mathematical model for the evolution of X chromosome inactivation in mammals (Engelstädter & Haig, 2008)

Why do we see such a huge diversity in genetic systems, sex determination and sex chromosome systems in nature? Our research tries to contribute to a better understanding of this question through mathematical models and computer simulations. For example, we have investigated how the spread of a gene causing parthenogenesis interacts with the existing sex determination system in the wasp species Lysephlebus fabarum (Engelstädter et al. 2011). We have also studied how Y chromosomes degenerate over time (Engelstädter 2008), and why one of the X chromosomes is inactivated in most female mammals (Engelstädter & Haig 2008). Finally, we have argued that one reason why parthenoegenesis is so rare in animals could be that it is quite difficult to abandon sex once it is a well-established mode of reproduction (Engelstädter 2008).
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Pessia, E., Engelstädter, J. & Marrais G.A.B.
The evolution of X chromosome inactivation in mammals: the demise of Ohno’s hypothesis?
Cellular and Molecular Life Sciences 71: 1383-1394

Engelstädter, J., Sandrock, C. & Vorburger, C.
Contagious parthenogenesis, automixis, and a sex determination meltdown.
Evolution 65: 501-511 (2011)

Engelstädter, J.
Constraints on the evolution of asexual reproduction.
BioEssays 30: 1138-1150 (2008)

Engelstädter, J.
Muller’s ratchet and the degeneration of Y chromosomes: A simulation study.
Genetics 180: 957-967 (2008)

Engelstädter, J. & Haig, D.
Sexual antagonism and the evolution of X chromosome inactivation.
Evolution 62: 2097-2104 (2008)

Engelstädter, J. & Hurst, G.D.D.
Can maternally transmitted endosymbionts facilitate the evolution of haplodiploidy?
Journal of Evolutionary Biology 19, 194-202 (2006)