Cover image for Vol. 38 Issue 6

Edited By: Andrew Moore

Online ISSN: 1521-1878

Science of Sex


The overwhelming majority of animals reproduce sexually. And whilst we ponder the evolutionary conundrum surrounding those that manage somehow without sex, we are still far from answering all the questions about the advantages of sex, the evolution of sex, and the nature of sexual differentiation and genetics. Below is a collection of articles from all corners of the enormous literature on sex. From primate behaviour in mate choice, through the genetics and epigenetics of sexual differentiation, to the molecular basis of sperm/egg recognition, sex is viewed from perspectives ranging from immunological fitness to speciation genetics. We wish you enjoyable reading.

For primer literature of relevance to the articles below, see the Encyclopedia of Life Sciences,ELS_Logo e.g. entry Mammalian Sex Determination.

The fragile Y hypothesis: Y chromosome aneuploidy as a selective pressure in sex chromosome and meiotic mechanism evolution
Heath Blackmon and Jeffery P. Demuth, BioEssays, Volume 37, Issue 9, September 2015, pages 942-950.
DOI: 10.1002/bies.201500040

achiasmatic meiosis, aneuploidy, fragile Y, sex chromosome evolution, Turner syndrome, Y-chromosome loss

The fragile Y hypothesis proposes that as Y-chromosomes degenerate aneuploidy rates increase, imposing an increasing mutational burden that is resolved in one of three fates: (i) temporary retention by rejuvenation; (ii) long-term retention by evolution of a meiotic mechanisms to ensure proper segregation without physical pairing; or (iii) complete Y loss.

The evolution of sex: A new hypothesis based on mitochondrial mutational erosion
Justin C. Havird, Matthew D. Hall and Damian K. Dowling, BioEssays, Volume 37, Issue 9, September 2015, pages 951-958.
DOI: 10.1002/bies.201500057

Also watch the Video Abstract.

cytonuclear, evolutionary genomics, mitochondrial replacement, mito-mutation accumulation, mito-nuclear interactions, organelle, recombination

Widespread sexual reproduction in eukaryotes remains an evolutionary enigma, because asexual reproduction should be favored by selection. We hypothesize that mutation accumulation in the mitochondria, an organelle inextricably linked to eukaryote evolution, would have favored the evolution of sex, because recombination would facilitate nuclear compensatory adaptation to offset mito-induced harm.

Wax, sex and the origin of species: Dual roles of insect cuticular hydrocarbons in adaptation and mating
Henry Chung and Sean B. Carroll, Volume 37, Issue 7, July 2015, pages 822-830.
DOI: 10.1002/bies.201500014

chemical communication, cuticular hydrocarbons, desiccation, ecological adaptation, mating success, speciation

Insect cuticular hydrocarbons (CHCs) play roles in both ecological adaptation as well as mate recognition. Because the synthesis of these molecules occur through a common biosynthesis pathway, the interplay between these two roles may offer an excellent model into understanding the molecular mechanisms underlying how ecological adaptation leads to the formation of new species.

How did viviparity originate and evolve? Of conflict, co-option, and cryptic choice
Alex T. Kalinka, Volume 37, Issue 7, July 2015, pages 721-731.
DOI: 10.1002/bies.201400200

cryptic female choice, Diptera, exaptation, genomic imprinting, postcopulatory sexual selection, sperm storage organs, viviparity

The image shows that postcopulatory sexual selection – sperm competition and cryptic female choice – favours retraction of the site of fertilization (SOF) within the female reproductive tract, and that this retraction is a necessary, but not sufficient, step to allow the evolution of viviparity.

Inclusive fitness and sexual conflict: How population structure can modulate the battle of the sexes
Tommaso Pizzari, Jay M. Biernaskie and Pau Carazo, Volume 37, Issue 2, February 2015, pages 155-166.
DOI: 10.1002/bies.201400130

cooperation, kin selection, sexual conflict, social evolution, tragedy of the commons

The struggle to reproduce pushes males to compete with each other, which often harms females in the process. Recent work proposes that, in structured populations, relatedness among local males should instead promote male cooperation and discourage female harm. Consequently, population structure should be considered to understand variation in sexual dynamics.

Did meiosis evolve before sex and the evolution of eukaryotic life cycles?
Karl J. Niklas, Edward D. Cobb and Ulrich Kutschera, Volume 36, Issue 11, November 2014, pages 1091-1101.
DOI: 10.1002/bies.201400045

algae, alternation of generation, auto-polyploidy, chiasmata, embryophytes, meiosis, syngamy

The evolution of meiosis is a major event in the history of life. A number of explanations for its evolution have been advanced. We revisit the proposition that meiosis evolved as an adaptation to whole-genome duplication (auto-polyploidy) and that it has been retained in most lineages to reduce sibling rivalry.

Variable escape from X-chromosome inactivation: Identifying factors that tip the scales towards expression
Samantha B. Peeters, Allison M. Cotton and Carolyn J. Brown, BioEssays, Volume 36, Issue 8, August 2014, pages 746-756.
DOI: 10.1002/bies.201400032

allelic imbalance, boundary elements, dosage compensation, epigenetic marks, RNA-seq, waystations, XIST

Incomplete transcriptional silencing of X-chromosome inactivation (XCI) results in some genes being variably expressed from the inactive X, with differences seen between females and between tissues. The variable expression likely reflects the impact of factors contributing to XCI, including DNA sequences, chromatin features, and 3D architecture of the chromosome.

Sex or no sex: Evolutionary adaptation occurs regardless
Michael F. Seidl and Bart P.H.J. Thomma, BioEssays, Volume 36, Issue 4, April 2014, pages 335-345.
DOI: 10.1002/bies.201300155

adaptation, asexual, genome evolution, meiosis, mitosis, recombination

Sex is a strong driver of genomic variation, which enables adaptation. Yet in many species, particularly in fungi, sex has never been observed. So how do presumed asexual organisms adapt? We hypothesize that asexuals evolved means to generate genomic diversity, mainly by genomic rearrangements, to compensate for the lack of sex.

A general mechanism for conditional expression of exaggerated sexually-selected traits
Ian A. Warren, Hiroki Gotoh, Ian M. Dworkin, Douglas J. Emlen and Laura C. Lavine, BioEssays, Volume 35, Issue 10, October 2013, pages 889-899.
DOI: 10.1002/bies.201300031

condition, exaggerated trait, insulin-like signaling pathway, sexual selection, signal trait

From the elaborate tails of peacocks to the enlarged head-horns of dung beetles, sexually-selected exaggerated traits are conspicuous and reliable signals of individual condition. How is the reliability and honesty of the signal maintained? We propose that co-option of the ancient and highly conserved insulin/insulin-like signaling pathway is the key.

Evolution of sex differences in lifespan and aging: Causes and constraints
Alexei A. Maklakov and Virpi Lummaa, BioEssays, Volume 35, Issue 8, August 2013, pages 717-724.
DOI: 10.1002/bies.201300021

aging, disease, mitochondria, senescence, sex chromosomes, sexual conflict, sexual selection

Why do sexes have different lifespans and aging rates? Sexes resolve the fundamental tradeoff between reproduction and survival differently and relationship between lifespan and fitness differs between males and females. However, selection for sex-specific values is constrained by intersexual genetic correlations resulting in intra-locus sexual conflict over optimal lifespan.

Both cell-autonomous mechanisms and hormones contribute to sexual development in vertebrates and insects
Ashley Bear and Antónia Monteiro, BioEssays, Volume 35, Issue 8, August 2013, pages 725-732.
DOI: 10.1002/bies.201300009

cell-autonomous, gonads, insects, sexual development, sexual dimorphism, sex hormones, vertebrates

Recent research has shown that the development of male and female traits in insects, birds, and mammals depends on both hormones and on the cell-autonomous expression of the sex determination pathway in somatic cells. This new research challenges traditional views of sexual development in birds, mammals, and insects.

Should Y stay or should Y go: The evolution of non-recombining sex chromosomes
Shenh Sun and Joseph Heitman, BioEssays, Volume 34, Issue 11, November 2012, 938-942.
DOI: 10.1002/bies.201200064

evolution, microorganism, sex chromosome

People might think that the evolutionary forces acting upon sex chromosomes and mating type loci would necessarily differ dramatically between multicellular animals and unicellular fungi, alga, or slime molds. However, a series of studies over the past decade have contributed to reveal a number of shared features among these systems.

Are old males still good males and can females tell the difference?
Sheri L. Johnson and Neil J. Gemmell, BioEssays, Volume 34, Issue 7, July 2012, 609-619.
DOI: 10.1002/bies.201100157

female mate preference, fertility, male age, mating history, sperm traits

Sperm function declines with male age, yet females of many species still choose to mate with older males. Do hidden advantages of mating with older males off-set costs related to fertility or can females detect declines in fertility and avoid mating with males that reduce offspring fitness?

Microbial manipulation of host sex determination
Leo W. Beukeboom, BioEssays, Volume 34, Issue 6, June 2012, 484-488.
DOI: 10.1002/bies.201100192

doublesex, endosymbiont, feminization, heterogamety, sex determination

Sex determination in insects is achieved through the action of a cascade of genes, and symbionts may interfere with their host at different levels of this cascade. One example is the lepidopteran Ostrinia scapulalis, where Wolbachia bacteria alter the sex-specific splicing of the doublesex master switch gene.

The male fight-flight response: A result of SRY regulation of catecholamines?
Joohyung Lee and Vincent R. Harley, BioEssays, Volume 34, Issue 6, June 2012, 454-457.
DOI: 10.1002/bies.201100159

sex chromosome, sex dimorphism, substantia nigra, sympathetic, tyrosine hydroxylase

The SRY gene, which is located on the Y chromosome and directs male development, may promote aggression and other traditionally male behavioural traits, resulting in the fight-or-flight reaction to stress.