Foreword: sex and sex chromosomes—new clues from nonmodel species

Abstract

IntroductionTheinvitationfromtheeditorsDrConlyRiederandProfHerbert Macgregor to edit this issue gave us the oppor-tunity to highlight some biological situations in whichsex chromosomes and sex determination are nonstan-dard, as the result of mutation or capricious evolution.Althoughtheymay,indeed,seemweird,thesenonmodelspecies provide us with the genetic variation that candeliver unique, and sometimes surprising, information.The focus of this issue is, therefore, how much wecan learn from such accidents of nature, comple-mented by variants produced to order. This issuetherefore contains articles on sex chromosomes inplants and insects as well as nonmodel vertebrates.The articles contain new data as well as new ideas,hypotheses and opinions, which we hope will attractcomment and reaction from readers in other fields. Wehave gathered articles on sex chromosomes and thegenes directly involved in sex determination but alsoincludedarticlesonsexdifferentiation,includinggeneticpathways in the development of gonads.Our other focus is on the extraordinarily rapidprogress that can be made using the tools of moderngenomics, even in quite bizarre organisms. Each of thesystems highlighted in this issue have had their loyaladherents, often over a very long time: for instance,the Y chromosome-less mole vole was first describedby Karl Fredga (1970), karyotypes of reptiles were allthe rage in the 1970s and bird sex determination hasbeen studied avidly for more than 50 years. However,without access to the armory of molecular techniques,many of these systems have been accessible only atthe cytological level.Sex chromosome organization and evolutionSex chromosomes are defined by a sex determininggene or genes, which trigger the genetic pathways thatcause undifferentiated gonads to differentiate intoeither ovary or testes. Depending on whether the sex-determining gene is male- or female-determining, sexchromosome systems are defined as XX female:XYmale (in which the heterogametic XY male producesX- or Y-bearing sperm) or ZZ male:ZW female (inwhich the heterogametic ZW female produces Z- or