Speciation in North American Junonia from a genomic perspective

Abstract

Delineating species boundaries in phylogenetic groups undergoing recent radiation is a daunting challenge akin to discretizing continuity. Here, we propose a general approach exemplified by American butterflies from the genus Junonia Hübner notorious for the variety of similar phenotypes, ease of hybridization, and the lack of consensus about their classification. We obtain whole‐genome shotgun sequences of about 200 specimens. We reason that discreteness emerges from continuity by means of a small number of key players, and search for the proteins that diverged markedly between sympatric populations of different species, while keeping low polymorphism within these species. Being 0.25% of the total number, these three dozen ‘speciation’ proteins indeed partition pairs of Junonia populations into two clusters with a prominent break in between, while all proteins taken together fail to reveal this discontinuity. Populations with larger divergence from each other, comparable to that between two sympatric species, form the first cluster and correspond to different species. The other cluster is characterized by smaller divergence, similar to that between allopatric populations of the same species and comprise conspecific pairs. Using this method, we conclude that J. genoveva (Cramer), J. litoralis Brévignon, J. evarete (Cramer), and J. divaricata C. & R. Felder are restricted to South America. We find that six species of Junonia are present in the United States, one of which is new: Junonia stemosa Grishin, sp.n. (i), found in south Texas and phenotypically closest to J. nigrosuffusa W. Barnes & McDunnough (ii) in its dark appearance. In the pale nudum of the antennal club, these two species resemble J. zonalis C. & R. Felder (iii) from Florida and the Caribbean Islands. The pair of sister species, J. grisea Austin & J. Emmel (iv) and J. coenia Hübner (v), represent the classic west/east U.S.A. split. The mangrove feeder (as caterpillar), dark nudum J. neildi Brévignon (vi) enters south Texas as a new subspecies Junonia neildi varia Grishin ssp.n. characterized by more extensive hybridization with and introgression from J. coenia, and, as a consequence, more variable wing patterns compared with the nominal J. n. neildi in Florida. Furthermore, a new mangrove‐feeding species from the Pacific Coast of Mexico is described as Junonia pacoma Grishin sp.n. Finally, genomic analysis suggests that J. nigrosuffusa may be a hybrid species formed by the ancestors of J. grisea and J. stemosa sp.n. This published work has been registered on Zoobank, http://zoobank.org/urn:lsid:zoobank.org:act:C6E70C96-463D-4E6A-95CC-B038.

A global molecular phylogeny yields insights into the dispersal and invasion history ofJunonia, a butterfly genus with remarkable dispersal abilities

“…Similarly, most New World Junonia species do not form monophyletic groups based on nuclear rRNA repeats. This is indicative of gene flow between New World Junonia species and is consistent with prior observations of hybridization between many of these species [ 20 , 23 , 30 , 32 ]. Based on the mitogenome phylogeny ( figure 2 ), the New World Junonia is monophyletic with two exceptions.…”

“…Species-level relationships in the New World Junonia were unresolved by mitogenomes ( figure 2 ), consistent with earlier DNA barcode and mitogenome studies [ 3 , 5 , 16 – 19 , 23 ]. Many New World species include individuals that carry both A and B haplotype groups.…”

“…Three primary mitochondrial haplotype groups are shared among New World Junonia species [ 3 , 6 , 9 , 16 – 20 , 23 ]. This pattern was first discussed by Pfeiler et al .…”

“…Recently, Cong et al . [ 23 ] analysed next-generation sequencing libraries from 11 New World Junonia species to describe two new species ( Junonia pacoma ; Pacific mangrove buckeye and Junonia stemosa ; South Texas dark buckeye). Fixed diagnostic characters (morphological or molecular) that allow for the consistent separation of J. stemosa from the morphologically nearly identical Junonia nigrosuffusa could not be identified , so we will treat this form as a subspecies J. nigrosuffusa stemosa nov. stat.…”

“…Fixed diagnostic characters (morphological or molecular) that allow for the consistent separation of J. stemosa from the morphologically nearly identical Junonia nigrosuffusa could not be identified , so we will treat this form as a subspecies J. nigrosuffusa stemosa nov. stat. [ 23 ]. To date, the largest mitogenome-based Junonia phylogeny consists of 28 mitogenomes (15 Junonia species (eight New World, seven Old World), and one from each of the five other Junoniini genera) [ 26 ].…”