Gene duplicates cause hybrid lethality between sympatric species ofMimulus

Abstract

12Hybrid incompatibilities play a critical role in the evolution and maintenance of species. We 13 have discovered a simple genetic incompatibility that causes lethality in hybrids between two 14 closely related species of yellow monkeyflower (Mimulus guttatus and M. nasutus). This hybrid 15 incompatibility, which causes one sixteenth of F 2 hybrid seedlings to lack chlorophyll and die 16 shortly after germination, occurs between sympatric populations that are connected by 17 ongoing interspecific gene flow. Using complimentary genetic mapping and gene expression 18 analyses, we show that lethality occurs in hybrids that lack a functional copy of the critical 19 photosynthetic gene pTAC14. In M. guttatus, this gene was duplicated, but the ancestral copy is 20 no longer expressed. In M. nasutus, the duplication is missing altogether. As a result, hybrids 21 die when they are homozygous for the nonfunctional M. guttatus copy and missing the 22 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/201392 doi: bioRxiv preprint first posted online 2 duplicate from M. nasutus, apparently due to misregulated transcription of key photosynthetic 23 genes. Our study indicates that neutral evolutionary processes may play an important role in 24 the evolution of hybrid incompatibilities and opens the door to direct investigations of their 25 contribution to reproductive isolation among naturally hybridizing species. 26 27 Author Summary 28Hybrid incompatibilities play an important role in speciation, because they act to limit gene flow 29 between species. Identifying the genes that underlie these barriers sheds light on the 30 evolutionary forces and genetic mechanisms that give rise to new species. We identified a 31 reproductive barrier that causes lethality in the F2 offspring of sympatric species of yellow 32 monkeyflower (Mimulus guttatus and M. nasutus). We show that lethality occurs in hybrids that 33 lack a functional copy of the critical photosynthetic gene pTAC14. This gene was duplicated in M. 34 guttatus, but the ancestral copy subsequently lost function. In M. nasutus, no duplication 35 occurred. As a consequence, F2 hybrids that are homozygous for non-functional M. guttatus 36 copies at one locus and missing M. nasutus duplicates at the other locus completely lack 37 functional pTAC14 and die. Our data indicate that non-functionalization of ancestral pTAC14 in 38 M. guttatus occurred via neutral evolutionary change. These results suggest that neutral 39 evolutionary forces may play an important role in speciation. 40 41

A two‐locus hybrid incompatibility is widespread, polymorphic, and active in natural populations of Mimulus *

“…In our original genetic characterization of the hl13‐hl14 incompatibility (Zuellig and Sweigart ), we used inbred lines derived from the DPR sympatric population of M. guttatus and M. nasutus . Given this fact, along with our previous discovery of interspecific gene flow at DPR (Brandvain et al.…”

“…), it is easy to imagine that ancestral polymorphism might exist at these loci. In our original genetic characterization of the hl13‐hl14 incompatibility, we showed that hybrid chlorosis occurred in seedlings that inherited M. guttatus alleles at hl13 , which carried nonfunctional versions of the ancestral copy of pTAC14 , along with M. nasutus alleles at hl14 , which lacked the duplicated copy of pTAC14 altogether (Zuellig and Sweigart ). We do not yet know the evolutionary timing of the pTAC14 duplication (onto hl14 ) or of the mutation that knocks out function of the ancestral copy on hl13 .…”

“…In the original cross between DPR102‐ gutt and DPR104‐ nas , white seedlings occurred at a frequency of one‐sixteenth in F2 hybrids (Fig. ; Zuellig and Sweigart ). In this study, if an F1 hybrid inherited an incompatible experimental allele at either hl13 or hl14 , one of the two experimental‐tester F2 progeny sets was expected to segregate white seedlings at a frequency of one‐sixteenth (Fig.…”

“…We extracted genomic DNA from seedlings using a cetyl trymethylammonium bromide (CTAB) chloroform protocol (Doyle ) modified for 96‐well format. Using a standard touchdown PCR protocol, we amplified fluorescently labeled markers linked to the hybrid sterility loci: M236 and M208 at hl13 , and M132 and M241 at hl14 as previously described (Zuellig and Sweigart ). Although markers were originally designed to flank the hybrid sterility loci, we later discovered that both M208 and M236 are proximal to hl13 .…”

“…The genes underlying hybrid lethality in Mimulus are now known (Zuellig and Sweigart ), although we do not yet have a straightforward, sequencing‐based approach to screen individuals for different pTAC14 variants (duplicate copies of pTAC14 are highly similar and distinguishing between them requires laborious cloning experiments). As an alternative, here we take a genetic crossing approach, generating F2 progeny between wild‐collected plants and tester lines that carry known genotypes at hl13 and hl14 .…”