many different genetic mechanisms of sex determination have been discovered in nature and studied deeply , using molecular - genetic tools ( reviewed in cline and meyer 1996 ; marin and baker 1998 ; zarkower 2001 ; williams and carroll 2009 ; charlesworth and mank 2010 ; gamble and zarkower 2012 ; hughes and rozen 2012 ) . in humans , a dominant male - determining gene is present on the y chromosome , whereas in drosophila melanogaster the dose of x chromosomes is the primary determinate of sex , and in c. elegans sex is determined by the ratio of x chromosomes to autosomes . the outcomes of these pathways are also different : males and females are the result in humans and d. melanogaster vs. males and hermaphrodites in c elegans . in the yeast saccharomyces cerevisiae ( reviewed in ni et al . 2011 ) diploid a/ organisms are derived by the fusion of opposite - sex , a and  haploids that are able to switch genetic material from one of two silent mating - type loci ( hmr and hml ) to the active mat locus through gene conversion initiated by the ho endonuclease . in plants that are obligate outcrossers , self - incompatibility ( si ) loci prevent productive self - fertilization , thereby effectively creating many mating types ( reviewed in barrett 2002 ) . ( 2014 ) examined the genetic basis of mating in the yeast zygosaccharomyces sapae , which typically lives in osmotically stressful environments , is often the cause of food spoilage and displays predominantly clonal reproduction . z. sapae is closely related to other species that reproduce sexually , leaving open questions of whether z. sapae has the genes that normally determine sex and what factors promoted the transition toward asexuality in this lineage . the z. sapae genome indeed contains genes homologous to those involved in mating - type determination in related yeast species , but it is unusual in containing three different versions of the  mating - type locus and two ho endonuclease genes , in addition to one version of the a mating - type locus . the identification of the a genotype leads the authors to propose that the stressful conditions in which this yeast normally finds itself could promote genome instability , with the sex chromosome perhaps being a hot spot for recombination and mutation . this genome instability in turn would produce mating - type imbalances that could disfavor or entirely prevent sexual reproduction , thereby contributing to genetic diversity in mating strategy among zygosaccharomyces yeasts . ( 2014 ) investigated the genetic and evolutionary basis of self - incompatibility in the plant tribe biscutelleae ( brassicaceae ) , which has a sporophytic self - incompatibility system ( ssi ) in which the pollen ( male gamete ) phenotype is determined by the diploid genotype of the paternal plant . the pollen typically expresses cysteine - rich proteins ( scr ) that are thought to interact with a transmembrane receptor ( srk ) produced on the pistil that receives the pollen . successful fertilization depends on different alleles being present at the s locus of pollen and pistils . ( 2014 ) characterized srk - like ( srkl ) sequences in biscutella neustriaca individuals from eight phenotypic incompatibility groups and discuss the association between srkl sequences and self - incompatibility phenotypes and the similarities and differences between biscutella and the better - studied but distantly related brassicaceae such as brassica and arabidopsis . sex chromosomes spend different amounts of time in males and females during an organism s evolution , usually experience less recombination than autosomes , and are often subject to dosage compensation . these differences lead to different selective pressures acting on sex - linked genes compared to the autosomal ones . taken further , as essential genes move from the sex chromosome to autosomes , sex chromosomes can be lost and replaced with new ones that evolve from ancestral autosomes . blackmon and demuth ( 2014 ) used phylogenetic analysis to reveal the tempo of y chromosome turnover in coleoptera . they analyzed karyotypes of 4724 beetle species that have heterogametic males that are xy , xo , or xy+ . xy+ are sex chromosome bivalents that display distance pairing ( no synapsis ) and thus do not recombine , allowing the authors to distinguish how differences in sex chromosome pairing and meiotic recombination influence the evolution of the sex chromosomes in two major beetle lineages , adephaga ( which have only xy and xo species ) , and polyphaga ( where over half of all species are xy+ ) . blackmon and demuth ( 2014 ) find that in adephaga , new y chromosomes are evolving at the same rate as they are lost , whereas in polyphaga y chromosomes are twice as readily gained as lost . the authors propose that different meiotic mechanisms that evolve to ensure sex chromosome segregation affect the tempo of y chromosome gain and loss . many sex chromosome pairs have a recombining region with largely similar gene content [ pseudoautosomal region ( par ) ] . kirkpatrick and guerrero ( 2014 ) use par to study sex - antagonistic ( sa ) selection . sa selection acts on alleles that are beneficial in one sex but detrimental to the other . sex chromosomes tend to bear more sa alleles than autosomes , presumably due to their sex - specific inheritance and limited recombination . this study uses coalescent models to show that sa selection , together with limited recombination , should lead to different patterns of neutral genetic variation on the x and y ( or z and w ) chromosomes . the authors test their model in silene latifolia , a plant with young recombining sex chromosomes , to illustrate their approach . the process of meiosis ensures that haploid complements of chromosomes segregate to each gamete through the processes of chromosome pairing , synapsis , and recombination . given that sex chromosomes are either not homologous along their entire length ( xy , zw ) or hemizygous ( xo , zo ) , additional mechanisms have evolved to ensure their segregation during meiosis . while homology - dependent mechanisms function in heterogametic organisms , it is not known how meiosis is altered in the hemizygous sex to ensure segregation of the single sex chromosome . furthermore , male and female gametes are morphologically distinct and undergo meiosis at different times during their developmental trajectories . 2014 examined the mechanisms that ensure segregation of the hemizygous x chromosome in c. elegans males ( xo ) . they demonstrate that double - strand dna breaks ( dsb ) are important for segregation of the hemizygous x and present evidence that dsbs could mediate sister chromatid synapsis ( pseudosynapsis ) , instead of the typical synapses formed between homologs . they go further and show that different genes function to provide feedback on autosomal synapsis and dsb repair pathways . they posit that these fundamental differences in meiotic processes evolved to accommodate hemizygosity of the x chromosome . zhang et al . ( 2014 ) examined the transcriptomes and proteomes during the differentiation of maize anthers , a model system well suited for this study because of the relative ease of collecting tissues at different developmental stages . in angiosperms , the germline arises from somatic flower cells through a switch from mitotic proliferation to meiosis . 2014 ) examined several stages on this developmental trajectory , from initial specification to final germinal and somatic cell differentiation . they describe transcriptome composition at different stages and show that many genes with meiotic functions are expressed prior to the onset of germ cell differentiation and meiosis . consistent with recent findings in other systems , they also find a low correlation between transcript abundance and protein abundance , suggesting a major role of post - transcriptional regulation in this developmental process . the inaugural collection highlights only some areas of sex - specific biology ; there are many remaining questions and discoveries to be made on a broad range of topics . this growing resource will illuminate the myriad ways in which sex differences influence biological processes .