Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth-death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

Sex Chromosome Evolution: So Many Exceptions to the Rules.

Nativist arguments Nativist " how else " arguments 9 Intricate system 9 Poverty of the stimulus-speed of acquisition (< 2 years)-age of the learner (< 3 years)-sensitive period (< 12 years) 9 Grammar is a set of abstract principles and a lexicon marked for the abstract syntactic principles 9 Acquisition of core grammar is accomplished in less than 2 years at a very young age 9 Grammatical principles are not learned, but are innate a priori's of the human brain (UG) The lexicon is learned, but the grammatical properties of the items are selected from UG How else but innate? (Pinker, Crain)

An Evolutionary Perspective

Dichotomy of Dosage Compensation along the Neo Z Chromosome of the Monarch Butterfly

SignificanceGenes on sex chromosomes (i.e. human chX) are regulated differently in males and females to balance gene expression levels between sexes (XY vs. XX). This sex-specific regulation is called dosage compensation (DC). DC is achieved by altering the shape and compaction of sex chromosomes specifically in one sex. In this study, we use Oligopaints to examine DC in silkworms. This study visualizes this phenomenon in a species with ZW sex chromosomes, which evolved independently of XY. Our data support a long-standing model for how DC mechanisms evolved across species, and we show potential similarity between DC in silkworms and nematodes, suggesting that this type of DC may have emerged multiple independent times throughout evolution. 

Dosage compensation in <i>Bombyx mori</i> is achieved by partial repression of both Z chromosomes in males

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

Sex chromosomal dose difference between sexes is often normalized by a gene regulatory mechanism called dosage compensation (DC). Studies indicate that DC mechanisms are generally effective in XY r...

/pdf/rna-sequencing-reveals-a-complete-but-an-unconventional-type-2tp6todibx.pdf

Kuchi Srikeerthana

Papers

RNA sequencing reveals a complete but an unconventional type of dosage compensation in the domestic silkworm Bombyx mori.