Losses of TMC and CIB neurosensory genes in schizophoran flies at the PETM

The K-Pg extinction ended the world of non-avian dinosaurs 66 Mya and ushered in the Age of Mammals. During the ~10 My of recovery, Schizophora, an immensely successful group of flies, appeared, flourished and diversified. This large radiation produced over half of all dipteran families (~78/150), including the family for Drosophila, the model genetic powerhouse. In the context of this evolutionary radiation, we investigate the loss of two of three highly conserved, neurosensory transmembrane channel (TMC) genes. Here, we show that these genes were separately lost across multiple diverging schizophoran lineages of the early Paleogene, suggesting a powerful environmental driver was involved. We also show that unlinked genes encoding the calcium- and integrin-binding (CIB) subunits of the missing TMC complexes were lost during the same time frame. Because the lost genes encode complexes involved in thermal nociception, we propose the external driver was likely the Paleocene-Eocene thermal maximum (PETM), a 200 ky interval of elevated global temperatures occurring 56 Mya, slightly before the earliest schizophoran fossil from 53 Mya. These results suggest that gene loss may have been adaptive for most schizophoran lineages to emerge past the hothouse Earth of the PETM.