A test of double interspecific introgression of nucleoporin genes in Drosophila

Comparative studies on speciation: 30 years since Coyne and Orr

Understanding the processes of population divergence and speciation remains a core question in evolutionary biology. For nearly a hundred years evolutionary geneticists have characterized reproductive isolation (RI) mechanisms and specific barriers to gene flow required for species formation. The seminal work of Coyne and Orr provided the first comprehensive comparative analysis of speciation. By combining phylogenetic hypotheses and species range data with estimates of genetic divergence and multiple mechanisms of RI across Drosophila, Coyne and Orr's influential meta-analyses answered fundamental questions and motivated new analyses that continue to push the field forward today. Now 30 years later, we revisit the five questions addressed by Coyne and Orr, identifying results that remain well supported and others that seem less robust with new data. We then consider the future of speciation research, with emphasis on areas where novel methods and data motivate potential progress. While the literature remains biased towards Drosophila and other model systems, we are enthusiastic about the future of the field.

Genetic Analyses of Elys Mutations in Drosophila Show Maternal-Effect Lethality and Interactions with Nucleoporin Genes

ELYS determines the subcellular localizations of Nucleoporins (Nups) during interphase and mitosis. We made loss-of-function mutations of Elys in Drosophila melanogaster and found that ELYS is dispensable for zygotic viability and male fertility but the maternal supply is necessary for embryonic development. Subsequent to fertilization, mitotic progression of the embryos produced by the mutant females is severely disrupted at the first cleavage division, accompanied by irregular behavior of mitotic centrosomes. The Nup160 introgression from D. simulans shows close resemblance to that of the Elys mutations, suggesting a common role for those proteins in the first cleavage division. Our genetic experiments indicated critical interactions between ELYS and three Nup107-160 subcomplex components; hemizygotes of either Nup37, Nup96 or Nup160 were lethal in the genetic background of the Elys mutation. Not only Nup96 and Nup160 but also Nup37 of D. simulans behave as recessive hybrid incompatibility genes with D. melanogaster An evolutionary analysis indicated positive natural selection in the ELYS-like domain of ELYS. Here we propose that genetic incompatibility between Elys and Nups may lead to reproductive isolation between D. melanogaster and D. simulans, although direct evidence is necessary.

Lineage-Specific Evolution of the Complex Nup160 Hybrid Incompatibility Between Drosophila melanogaster and Its Sister Species

Two genes encoding protein components of the nuclear pore complex Nup160 and Nup96 cause lethality in F2-like hybrid genotypes between Drosophila simulans and Drosophila melanogaster. In particular, D. simulans Nup160 and Nup96 each cause inviability when hemizygous or homozygous in species hybrids that are also hemizygous (or homozygous) for the D. melanogaster X chromosome. The hybrid lethality of Nup160, however, is genetically complex, depending on one or more unknown additional factors in the autosomal background. Here we study the genetics and evolution of Nup160-mediated hybrid lethality in three ways. First, we test for variability in Nup160-mediated hybrid lethality within and among the three species of the D. simulans clade- D. simulans, D. sechellia, and D. mauritiana. We show that the hybrid lethality of Nup160 is fixed in D. simulans and D. sechellia but absent in D. mauritiana. Second, we explore how the hybrid lethality of Nup160 depends on other loci in the autosomal background. We find that D. simulans Nup160-mediated hybrid lethality does not depend on the presence of D. melanogaster Nup96, and we find that D. simulans and D. mauritiana are functionally differentiated at Nup160 as well as at other autosomal factor(s). Finally, we use population genetics data to show that Nup160 has experienced histories of recurrent positive selection both before and after the split of the three D. simulans clade species ∼240,000 years ago. Our genetic results suggest that a hybrid lethal Nup160 allele evolved before the split of the three D. simulans clade species, whereas the other autosomal factor(s) evolved more recently.