Draft:Neo-Sex Chromosomes

The term “neo-sex chromosome” refers to a sex chromosomes produced by chromosomal fusion or translocation. A neo-sex chromosome is formed when either segments or the entirety of an autosome fuses to a preexisting sex chromosome. The addition of new genetic data to a sex chromosome can facilitate rapid and dramatic changes in the phenotype of an organism. Modern, unaltered sex chromosomes are generally highly dimorphic. Neo-sex chromosomes can be more similar to one another, or hold more genetic information. Studying recently formed neo-sex chromosomes provides insight into sex chromosome evolution.

Through complete Chromosomal fusion
Chromosomal fusion refers to the complete conjoining of two chromosomes, typically from end-to-end through the telomeres, leading to the synthesis of a new, larger chromosome containing the genetic information of both. This process is relatively rare and characteristically always leads to a reduction in the organism's number of chromosomes, changing the karyotype. In this scenario, one of the ancestral sex chromosomes will have had to have been completely lost in this process so as not to lead to monosomy, or species-wide aneuploidy. The distinction between chromosomal translocation and chromosomal fusion lies in that two chromosomes join completely to form a new one, as well as the fact that end-to-end fusions do not involve the swapping of segments. Autosomes may fuse to sex chromosomes in this manner to create neo-sex chromosomes.

Through Translocation
Translocation is the fusion of chromosomal information, typically between two non-homologous chromosomes, where a genetic abnormality causes a segment of the chromosomes to break off and reattach to each other. It can occur reciprocally, where two chromosomes exchange segments and no genetic information is lost; or non-reciprocally, where one chromosome segment is transferred to another unbroken chromosome, often leading to harmful gene deletions or gene duplication. Both occurrences could hypothetically lead to the integration of genetic information from autosomal chromosomes into sex chromosomes, leading to a neo-sex chromosome.

Through Robertsonian Translocation
Robertsonian translocation (otherwise referred to as centric fusion) refers to the merging of two chromosomes via the centromeres. In this instance the shorter arms of the resulting chromosome may fall off, but the genetic information lost is not dangerous. Centric fusion classifies as both fusion and translocation because it simultaneously results in a reduction in the karyotype and involves swapping of segments through the centromere. This has been a documented mechanism of neo-sex chromosomes.

Relationship to Evolutionary Mechanisms
The proliferation of neo-sex chromosomes could require extreme selective pressure, or simply fixate through genetic drift. Chromosomal combination of any kind is almost always initiated by mutation, and a species or population-wide change in chromosome could only result from some combination of natural selection, genetic drift, non-random mating, and gene flow. Additionally, the chances of a neo-sex chromosome spreading at all relies on its natural fitness, which could only arise in a select few instances of autosomal and sex chromosome fusion.

Significance to heterogametic sex chromosomes
The basic or traditional models of sex chromosome evolution highlight a rapid loss of genetic complexity and material of heterogametic sex chromosomes, leading to an assumption that the degeneration of these chromosomes is inevitable. The two prevalent sex-determination systems are the XY and ZW systems. In XY systems, the homogametic XX typically determines female sex, and the heterogametic XY male. In ZW, the inverse is true, with homozygous ZZ signaling male sex and ZW signaling female. Due to the differences between heterogametic sex chromosome pairs such as the XY pairing, they are unable to undergo genetic recombination during reproduction seen in autosomal chromosomes and homogametic sex chromosome pairings like XX. Since genetic recombination in homologous chromosomes serves to eliminate damaging gene mutations, heterogametic sex chromosomes are unable to combat deleterious genes, making them progressively degenerate over generations. This pattern is most well documented in the degeneration of the human y chomosome, but there is also an abundance of information on degeneration in bird species’ (female heterogametic) W chromosome. When a neo-sex chromosome forms, the process of degeneration can be reset. The formation of one on a heterogametic chromosome signifies a sudden increase in its genotypic content, one of the greatest examples being the Crested Ibis, a member of the Aequornithes (core waterbird) family.

In Kiwifruit
Neo-Sex chromosomes have evolved recurrently and independently through an unspecified form of translocation in Kiwifruit. The Kiwifruit is dioecious and highly dependent on having male and female plants. The resulting changes in the species has been linked to the maintenance of sexual dimorphism, enhanced male traits that increase pollen distribution and reduce reproduction time, and increased adaptability through facilitating rapid evolution.

In Grasshoppers
Neo-Sex chromosomes arose in the grasshopper through centric fusion of an autosomal chromosome to the X chromosome.

In Drosophila
Three independent instances of neo-sex chromosomes arising in Drosophila (fruit flies). Attributed to solving sexual conflict.

In The Crested Ibis
The Crested Ibis is a rare example of slow-degradation of the heterogametic chromosome. The Ibis underwent a fusion of autosomal microchromosomes to an ancient W sex-chomosome, creating a neo-sex chromosome. This effectively interrupted the process of expected degeneration of the W chromosome. It was found that certain retained genes present in the W chromosome of female Crested Ibis have been lost by other species. These genes are linked to female-biased gene-expression, a motivation for sexual selection that had previously been ruled out in bird species. This preservation is likely due in part to the addition of the neo-sex chromosome, allowing deleterious genes to interact with non-ancient information. Additionally, the Crested Ibis has physical characteristics that also are linked to slow-degradation, though these may or may not have a relationship with the neo-chromosome.