Speciation Generates Mosaic Genomes in Kangaroos

Genomic divergence and introgression between cryptic species of a widespread North American songbird

Analysis of genomic variation among related populations can sometimes reveal distinct species that were previously undescribed due to similar morphological appearances, and close examination of such cases can provide much insight regarding speciation. Genomic data can also reveal the role of reticulate evolution in differentiation and speciation. White-breasted nuthatches (Sitta carolinensis) are widely distributed North American songbirds that are currently classified as a single species but have been suspected to represent a case of cryptic speciation. Previous genetic analyses suggested four divergent groups, but it was unclear whether these represented multiple reproductively isolated species. Using extensive genomic sampling of over 350 white-breasted nuthatches from across North America and a new chromosome-level reference genome, we asked if white-breasted nuthatches are comprised of multiple species and whether introgression has occurred between divergent populations. Genomic variation of over 300,000 loci revealed four highly differentiated populations (Pacific, n = 45; Eastern, n = 23; Rocky Mountains North, n = 138; and Rocky Mountains South, n = 150) with geographic ranges that are adjacent. We observed a moderate degree of admixture between Rocky Mountain populations but only a small number of hybrids between the Rockies and the Eastern population. The rarity of hybrids together with high levels of differentiation between populations is supportive of populations having some level of reproductive isolation. Between populations, we show evidence for introgression from a divergent ghost lineage of white-breasted nuthatches into the Rocky Mountains South population, which is otherwise closely related to Rocky Mountains North. We conclude that white-breasted nuthatches are best considered at least three species and that ghost lineage introgression has contributed to differentiation between the two Rocky Mountain populations. White-breasted nuthatches provide a dramatic case of morphological similarity despite high genomic differentiation, and the varying levels of reproductive isolation among the four groups provide an example of the speciation continuum.

Characterization of the marsupial endogenous retrovirus walb with a focus on satellite DNA formation

The walbRep megasatellite DNA found in the red-necked wallaby was formed from the walb endogenous retrovirus. Our previous PCR experiments suggested the presence of walb and absence of walbRep in the genome of the tammar wallaby, which diverged from the red-necked wallaby 2-3 Mya. The results failed to exclude the possibility that certain walbRep sequences might have remained undetected owing to variation in the primer-annealing regions; therefore, the aforementioned suggestion was not confirmed. To obtain conclusive evidence, we analyzed the structure of walb sequences drawn from the tammar wallaby genome database recently updated to a chromosome-level assembly. All walb copies existed as separate DNA segments, not constituting tandem repeats. We concluded that walbRep was formed in the red-necked wallaby lineage after its divergence from the tammar wallaby. We also confirm the presence of a walb copy with an anomalistic, complex structure and propose a simple model for its generation mechanism.

Estimation of species divergence times in presence of cross-species gene flow

Cross-species introgression can have significant impacts on phylogenomic reconstruction of species divergence events. Here, we used simulations to show how the presence of even a small amount of introgression can bias divergence time estimates when gene flow is ignored in the analysis. Using advances in analytical methods under the multispecies coalescent (MSC) model, we demonstrate that by accounting for incomplete lineage sorting and introgression using large phylogenomic data sets this problem can be avoided. The multispecies-coalescent-with-introgression (MSci) model is capable of accurately estimating both divergence times and ancestral effective population sizes, even when only a single diploid individual per species is sampled. We characterize some general expectations for biases in divergence time estimation under three different scenarios: 1) introgression between sister species, 2) introgression between non-sister species, and 3) introgression from an unsampled (i.e., ghost) outgroup lineage. We also conducted simulations under the isolation-with-migration (IM) model and found that the MSci model assuming episodic gene flow was able to accurately estimate species divergence times despite high levels of continuous gene flow. We estimated divergence times under the MSC and MSci models from two published empirical datasets with previous evidence of introgression, one of 372 target-enrichment loci from baobabs (Adansonia), and another of 1000 transcriptome loci from 14 species of the tomato relative, Jaltomata. The empirical analyses not only confirm our findings from simulations, demonstrating that the MSci model can reliably estimate divergence times but also show that divergence time estimation under the MSC can be robust to the presence of small amounts of introgression in empirical datasets with extensive taxon sampling. [divergence time; gene flow; hybridization; introgression; MSci model; multispecies coalescent].

The genome of the pygmy right whale illuminates the evolution of rorquals

BACKGROUND

Baleen whales are a clade of gigantic and highly specialized marine mammals. Their genomes have been used to investigate their complex evolutionary history and to decipher the molecular mechanisms that allowed them to reach these dimensions. However, many unanswered questions remain, especially about the early radiation of rorquals and how cancer resistance interplays with their huge number of cells. The pygmy right whale is the smallest and most elusive among the baleen whales. It reaches only a fraction of the body length compared to its relatives and it is the only living member of an otherwise extinct family. This placement makes the pygmy right whale genome an interesting target to update the complex phylogenetic past of baleen whales, because it splits up an otherwise long branch that leads to the radiation of rorquals. Apart from that, genomic data of this species might help to investigate cancer resistance in large whales, since these mechanisms are not as important for the pygmy right whale as in other giant rorquals and right whales.

RESULTS

Here, we present a first de novo genome of the species and test its potential in phylogenomics and cancer research. To do so, we constructed a multi-species coalescent tree from fragments of a whole-genome alignment and quantified the amount of introgression in the early evolution of rorquals. Furthermore, a genome-wide comparison of selection rates between large and small-bodied baleen whales revealed a small set of conserved candidate genes with potential connections to cancer resistance.

CONCLUSIONS

Our results suggest that the evolution of rorquals is best described as a hard polytomy with a rapid radiation and high levels of introgression. The lack of shared positive selected genes between different large-bodied whale species supports a previously proposed convergent evolution of gigantism and hence cancer resistance in baleen whales.

Marsupial genome analysis suggests that satellite DNA formation from walb endogenous retrovirus is an event specific to the red-necked wallaby

We recently identified walbRep, a satellite DNA residing in the genome of the red-necked wallaby Notamacropus rufogriseus. It originates from the walb endogenous retrovirus and is organized in a manner in which the provirus structure is retained. The walbRep repeat units feature an average pairwise nucleotide identity as high as 99.5%, raising the possibility of a recent origin. The tammar wallaby N. eugenii is a species estimated to have diverged from the red-necked wallaby 2-3 million years ago. In PCR analyses of these two and other related species, walbRep-specific fragment amplification was observed only in the red-necked wallaby. Sequence database searches for the tammar wallaby resulted in sequence alignment lists that were sufficiently powerful to exclude the possibility of walbRep existence. These results suggested that the walbRep formation occurred in the red-necked wallaby lineage after its divergence from the tammar wallaby lineage, thus in a time span of maximum 3 million years.

Marsupial satellite DNA as faithful reflections of long terminal repeat (LTR) retroelement structure

Long terminal repeat (LTR) retroelements, including endogenous retroviruses, are one of the origins of satellite DNAs. However, the vast majority of satellite DNAs originating from LTR retroelements consist of parts of the element. In addition, they frequently contain sequences unrelated to that element. Here we report a novel marsupial satellite DNA (named walbRep) that contains, and consists solely of, the entire sequence of an LTR retroelement (the walb element). As is common with LTR retroelements, walb copies exhibit length variation. We focused on the abundance of copies of a specific length (2.7 kb) in the genome of the red-necked wallaby. Cloning and analyses of long genomic DNA fragments revealed a satellite DNA in which the LTR sequence (0.4 kb) and the sequence of the internal region of a nonautonomous walb copy (2.3 kb) were repeated alternately. The junctions between these two components exhibited the same end-to-end arrangements as those in the walb element. This satellite organization could be accounted for by a simple formation model that includes slippage during chromosome pairing followed by homologous recombination but does not invoke any other types of rearrangements. We discuss the possible reasons why satellite DNAs having such structures are rarely found in mammals.

distAngsd: Fast and accurate inference of genetic distances for Next Generation Sequencing data

Commonly used methods for inferring phylogenies were designed before the emergence of high throughput sequencing and can generally not accommodate the challenges associated with noisy, diploid sequencing data. In many applications, diploid genomes are still treated as haploid through the use of ambiguity characters; while the uncertainty in genotype calling - arising as a consequence of the sequencing technology - is ignored. In order to address this problem we describe two new probabilistic approaches for estimating genetic distances: distAngsd-geno and distAngsd-nuc, both implemented in a software suite named distAngsd. These methods are specifically designed for next generation sequencing data, utilize the full information from the data, and take uncertainty in genotype calling into account. Through extensive simulations, we show that these new methods are markedly more accurate and have more stable statistical behaviors than other currently available methods for estimating genetic distances - even for very low depth data with high error rates.

Incomplete lineage sorting and phenotypic evolution in marsupials

Incomplete lineage sorting (ILS) makes ancestral genetic polymorphisms persist during rapid speciation events, inducing incongruences between gene trees and species trees. ILS has complicated phylogenetic inference in many lineages, including hominids. However, we lack empirical evidence that ILS leads to incongruent phenotypic variation. Here, we performed phylogenomic analyses to show that the South American monito del monte is the sister lineage of all Australian marsupials, although over 31% of its genome is closer to the Diprotodontia than to other Australian groups due to ILS during ancient radiation. Pervasive conflicting phylogenetic signals across the whole genome are consistent with some of the morphological variation among extant marsupials. We detected hundreds of genes that experienced stochastic fixation during ILS, encoding the same amino acids in non-sister species. Using functional experiments, we confirm how ILS may have directly contributed to hemiplasy in morphological traits that were established during rapid marsupial speciation ca. 60 mya.

A 16S Next Generation Sequencing Based Molecular and Bioinformatics Pipeline to Identify Processed Meat Products Contamination and Mislabelling

Simple Summary

Meat adulteration and fraud encompasses the deliberate fraudulent addition or substitution of proteins of animal or plant origin in edible products primarily for economic gain. The mitochondrial 16S ribosomal (rRNA) gene was used to identify species that are present in pure and processed meat samples. The meat samples were sequenced using an Illumina sequencing platform, and bioinformatics analysis was carried out for species identification. The results indicated that pork was the major contaminant in most of the meat samples. The bioinformatics pipeline demonstrated its specificity through identification of species specific and quantification of the contamination levels across all samples. Food business operators and regulatory sectors can validate this method for food fraud checks and manage any form of mislabeling in the animal or plant protein food ecosystem.

Abstract

Processed meat is a target in meat adulteration for economic gain. This study demonstrates a molecular and bioinformatics diagnostic pipeline, utilizing the mitochondrial 16S ribosomal RNA (rRNA) gene, to determine processed meat product mislabeling through Next-Generation Sequencing. Nine pure meat samples were collected and artificially mixed at different ratios to verify the specificity and sensitivity of the pipeline. Processed meat products (n = 155), namely, minced meat, biltong, burger patties, and sausages, were collected across South Africa. Sequencing was performed using the Illumina MiSeq sequencing platform. Each sample had paired-end reads with a length of ±300 bp. Quality control and filtering was performed using BBDuk (version 37.90a). Each sample had an average of 134,000 reads aligned to the mitochondrial genomes using BBMap v37.90. All species in the artificial DNA mixtures were detected. Processed meat samples had reads that mapped to the Bos (90% and above) genus, with traces of reads mapping to Sus and Ovis (2–5%) genus. Sausage samples showed the highest level of contamination with 46% of the samples having mixtures of beef, pork, or mutton in one sample. This method can be used to authenticate meat products, investigate, and manage any form of mislabeling.

pgHMA: Application of the heteroduplex mobility assay analysis in phylogenetics and population genetics

The heteroduplex mobility assay (HMA) has proven to be a robust tool for the detection of genetic variation. Here, we describe a simple and rapid application of the HMA by microfluidic capillary electrophoresis, for phylogenetics and population genetic analyses (pgHMA). We show how commonly applied techniques in phylogenetics and population genetics have equivalents with pgHMA: phylogenetic reconstruction with bootstrapping, skyline plots, and mismatch distribution analysis. We assess the performance and accuracy of pgHMA by comparing the results obtained against those obtained using standard methods of analyses applied to sequencing data. The resulting comparisons demonstrate that: (a) there is a significant linear relationship (R²  = .992) between heteroduplex mobility and genetic distance, (b) phylogenetic trees obtained by HMA and nucleotide sequences present nearly identical topologies, (c) clades with high pgHMA parametric bootstrap support also have high bootstrap support on nucleotide phylogenies, (d) skyline plots estimated from the UPGMA trees of HMA and Bayesian trees of nucleotide data reveal similar trends, especially for the median trend estimate of effective population size, and (e) optimized mismatch distributions of HMA are closely fitted to the mismatch distributions of nucleotide sequences. In summary, pgHMA is an easily-applied method for approximating phylogenetic diversity and population trends.

Gene Flow and Diversification in Himalopsyche martynovi Species Complex (Trichoptera: Rhyacophilidae) in the Hengduan Mountains

The Hengduan Mountains are one of the most species-rich mountainous areas in the world. The origin and evolution of such a remarkable biodiversity are likely to be associated with geological or climatic dynamics, as well as taxon-specific biotic processes (e.g., hybridization, polyploidization, etc.). Here, we investigate the mechanisms fostering the diversification of the endemic Himalopsyche martynovi complex, a poorly known group of aquatic insects. We used multiple allelic datasets generated from 691 AHE loci to reconstruct species and RaxML phylogenetic trees. We selected the most reliable phylogenetic tree to perform network and gene flow analyses. The phylogenetic reconstructions and network analysis identified three clades, including H. epikur, H. martynovi sensu stricto and H. cf. martynovi. Himalopsyche martynovi sensu stricto and H. cf. martynovi present an intermediate morphology between H. epikur and H. viteceki, the closest known relative to the H. martynovi-complex. The gene flow analysis revealed extensive gene flow among these lineages. Our results suggest that H. viteceki and H. epikur are likely to have contributed to the evolution of H. martynovi sensu stricto and H. cf. martynovi via gene flow, and thus, our study provides insights in the diversification process of a lesser-known ecological group, and hints at the potential role of gene flow in the emergence of biological novelty in the Hengduan Mountains.

Enhancing mitogenomic phylogeny and resolving the relationships of extinct megafaunal placental mammals

Mitochondrial genomes provided the first widely used sequences that were sufficiently informative to resolve relationships among animals across a wide taxonomic domain, from within species to between phyla. However, mitogenome studies supported several anomalous relationships and fell partly out of favour as sequencing multiple, independent nuclear loci proved to be highly effective. A tendency to blame mitochondrial DNA (mtDNA) has overshadowed efforts to understand and ameliorate underlying model misspecification. Here we find that influential assessments of the infidelity of mitogenome phylogenies have often been overstated, but nevertheless, substitution saturation and compositional non-stationarity substantially mislead reconstruction. We show that RY coding the mtDNA, excluding protein-coding 3rd codon sites, partitioning models based on amino acid hydrophobicity and enhanced taxon sampling improve the accuracy of mitogenomic phylogeny reconstruction for placental mammals, almost to the level of multi-gene nuclear datasets. Indeed, combined analysis of mtDNA with 3-fold longer nuclear sequence data either maintained or improved upon the nuclear support for all generally accepted clades, even those that mtDNA alone did not favour, thus indicating "hidden support". Confident mtDNA phylogeny reconstruction is especially important for understanding the evolutionary dynamics of mitochondria themselves, and for merging extinct taxa into the tree of life, with ancient DNA often only accessible as mtDNA. Our ancient mtDNA analyses lend confidence to the relationships of three extinct megafaunal taxa: glyptodonts are nested within armadillos, the South American ungulate, Macrauchenia is sister to horses and rhinoceroses, and sabre-toothed and scimitar cats are the monophyletic sister-group of modern cats.

Multiplex PCRs for the specific identification of marsupial and deer species from faecal samples as a basis for non-invasive epidemiological studies of parasites

Background

The specific identification of animals through the analysis of faecal DNA is important in many areas of scientific endeavour, particularly in the field of parasitology.

Methods

Here, we designed and assessed two multiplex PCR assays using genetic markers in a mitochondrial cytochrome b (cytb) gene region for the unequivocal identification and discrimination of animal species based on the specific amplification of DNA from faecal samples collected from water catchment areas in Victoria, Australia. One of these assays differentiates three marsupial species (eastern grey kangaroo, swamp wallaby and common wombat) and the other distinguishes three deer species (fallow, red and sambar deer). We tested these two assays using a total of 669 faecal samples, collected as part of an ongoing programme to monitor parasites and microorganisms in these animals.

Results

These two PCR assays are entirely specific for these animal species and achieve analytical sensitivities of 0.1–1.0 picogram (pg). We tested 669 faecal samples and found that some previous inferences of species based on faecal morphology were erroneous. We were able to molecularly authenticate all of the 669 samples.

Conclusions

We have established PCR assays that accurately distinguish the faecal samples of some of the prominent large mammalian herbivores found within a water catchment system in the state of Victoria, Australia. The multiplex assays for marsupials and deer produce amplicons that are easily differentiable based on their size on an agarose gel, and can be readily sequenced for definitive species authentication. Although established for marsupials and deer, the methodology used here can be applied to other host-parasite study systems to ensure data integrity.

Identification of a novel hybrid zone within the black-footed rock-wallaby (Petrogale lateralis) in Western Australia

It is increasingly recognised that intertaxon hybridisation is more common in vertebrates than previously thought. However, recent hybridisation has rarely been reported from wild marsupials, with only three instances of first generation (F1) hybrids reported, all in macropodids. In the 1990s a chromosomally anomalous population of black-footed rock-wallaby (Petrogale lateralis) was identified in the Townsend Ridges in central eastern Western Australia. Individuals from this population had chromosomes characteristic of two P. lateralis subspecies (P. l. centralis and P. l. kimberleyensis). This unusual mixture is suggestive of a novel hybrid zone between subspecies, but it could also represent a P. l. centralis population in which a 9–10 chromosome fusion has independently arisen. To test between these hypotheses, we compared mitochondrial DNA Control Region (CR) sequence data from Townsend Ridges individuals to published data for all P. lateralis subspecies. Two divergent lineages of CR haplotypes were identified at Townsend Ridges, suggesting that it represents a novel rock-wallaby hybrid zone, the third reported in the genus. While one CR haplotype clustered with those typical of P. l. centralis, the other Townsend Ridges haplotypes clustered with those from three different P. lateralis subspecies but not with P. l. kimberleyensis. Additional studies with multiple nuclear genes will be necessary to fully understand the nature of this novel hybrid zone.

Phylogenetic relationships of the cuscuses (Diprotodontia : Phalangeridae) of island Southeast Asia and Melanesia based on the mitochondrial ND2 gene

The species-level systematics of the marsupial family Phalangeridae, particularly Phalanger, are poorly understood, due partly to the family’s wide distribution across Australia, New Guinea, eastern Indonesia, and surrounding islands. In order to refine the species-level systematics of Phalangeridae, and improve our understanding of their evolution, we generated 36 mitochondrial ND2 DNA sequences from multiple species and sample localities. We combined our new data with available sequences and produced the most comprehensive molecular phylogeny for Phalangeridae to date. Our analyses (1) strongly support the monophyly of the three phalangerid subfamilies (Trichosurinae, Ailuropinae, Phalangerinae); (2) reveal the need to re-examine all specimens currently identified as ‘Phalanger orientalis’; and (3) suggest the elevation of the Solomon Island P. orientalis subspecies to species level (P. breviceps Thomas, 1888). In addition, samples of P. orientalis from Timor formed a clade, consistent with an introduction by humans from a single source population. However, further research on east Indonesian P. orientalis populations will be required to test this hypothesis, resolve inconsistencies in divergence time estimates, and locate the source population and taxonomic status of the Timor P. orientalis.

Rapid Pliocene adaptive radiation of modern kangaroos

Differentiating between ancient and younger, more rapidly evolved clades is important for determining paleoenvironmental drivers of diversification. Australia possesses many aridity-adapted lineages, the origins of which have been closely linked to late Miocene continental aridification. Using dental macrowear and molar crown height measurements, spanning the past 25 million years, we show that the most iconic Australian terrestrial mammals, “true” kangaroos (Macropodini), adaptively radiated in response to mid-Pliocene grassland expansion rather than Miocene aridity. In contrast, low-crowned, short-faced kangaroos radiated into predominantly browsing niches as the late Cenozoic became more arid, contradicting the view that this was an interval of global browser decline. Our results implicate warm-to-cool climatic oscillations as a trigger for adaptive radiation and refute arguments attributing Pleistocene megafaunal extinction to aridity-forced dietary change.