One feverish night, just over 100 years ago, an undergraduate in Thomas Hunt Morgan’s lab created the first genetic map. Realizing that the frequency of crossing over could be used to work out out the linear order of genes on a chromosome, that student, Alfred Sturtevant, published his map in 1913 and laid the foundation for a century of genome analysis. Today, even with the rise of rapid sequencing methods, meiotic maps still play a critical role in assembling whole genome sequences and tracing the evolution of genomes. In this month’s issue of GENETICS, Amores et al. use meiotic mapping to reveal some surprising features of genome evolution in fish.
Chromosome numbers in teleost fish are relatively stable compared to mammals. For example, some deer species have 40 chromosomes, while others have just three. In contrast, 58% of teleosts have either 24 or 25 chromosomes. To explore this karyotype stability, the authors made an extensive RAD-tag genetic map for platyfish (Xiphophorus maculatus) and aligned the map to contigs and scaffolds from the platyfish reference genome, identifying likely assembly errors in the process. By comparing the results with other fish species, they found a striking stability of chromosome gene content.
For example, several entire chromosomes are maintained intact between zebrafish and platyfish, whose lineages separated around 300 million years ago. The authors also showed that about 80% of platyfish chromosomes have a one-to-one relationship with medaka fish chromosomes, even though they have been diverging for 120 million years. In contrast, mouse and human lineages diverged only about 75 million years ago, but only their X chromosomes show a one-to-one relationship. The work also reveals how reduced chromosome numbers in stickleback and green pufferfish arose independently by fusion of pairs of ancestral chromosomes after their lineages diverged from platyfish.
From Sturtevant’s simple diagram of six Drosophila “factors,” to a platyfish map of over 16,000 markers, meiotic maps have grown larger and more detailed, but remain powerful tools for understanding how genomes function and evolve. Read the article.
A RAD-Tag Genetic Map for the Platyfish (Xiphophorus maculatus) Reveals Mechanisms of Karyotype Evolution Among Teleost Fish Angel Amores, Julian Catchen, Indrajit Nanda, Wesley Warren, Ron Walter, Manfred Schartl, and John H. Postlethwait Genetics June 2014 197:625-641 doi:10.1534/genetics.114.164293