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We tested the hypothesis that the rate of marsupial cranial evolution is dependent on the distribution of genetic variation in multivariate space. To do so, we carried out a genetic analysis of cranial morphological variation in laboratory strains of Monodelphis domestica and used estimates of genetic covariation to analyze the morphological diversification of the Monodelphis brevicaudata species group. We found that within-species genetic variation is concentrated in only a few axes of the morphospace and that this strong genetic covariation influenced the rate of morphological diversification of the brevicaudata group, with between-species divergence occurring fastest when occurring along the genetic line of least resistance. Accounting for the geometric distribution of genetic variation also increased our ability to detect the selective regimen underlying species diversification, with several instances of selection only being detected when genetic covariances were taken into account. Therefore, this work directly links patterns of genetic covariation among traits to macroevolutionary patterns of morphological divergence. Our findings also suggest that the limited distribution of Monodelphis species in morphospace is the result of a complex interplay between the limited dimensionality of available genetic variation and strong stabilizing selection along two major axes of genetic variation.

Background: Tree squirrels (Sciuridae, Sciurini), in particular the highly diverse Neotropical lineages, are amongst the most rapidly diversifying branches of the mammal tree of life but also some of the least known. Negligence of this group by systematists is likely a product of the difficulties in assessing morphological informative traits and of the scarcity or unavailability of fresh tissue samples for DNA sequencing. The highly discrepant taxonomic arrangements are a consequence of the lack of phylogenies and the exclusive phenotypic-based classifications, which can be misleading in a group with conservative morphology. Here we used high-throughput sequencing and an unprecedented sampling of museum specimens to provide the first comprehensive phylogeny of tree squirrels, with a special emphasis on Neotropical taxa. Results: We obtained complete or partial mitochondrial genomes from 232 historical and modern samples, representing 40 of the 43 currently recognized species of Sciurini. Our phylogenetic analyses-performed with datasets differing on levels of missing data and taxa under distinct analytical methods-strongly support the monophyly of Sciurini and consistently recovered 12 major clades within the tribe. We found evidence that the diversity of Neotropical tree squirrels is underestimated, with at least six lineages that represent taxa to be named or revalidated. Ancestral state reconstructions of number of upper premolars and number of mammae indicated that alternative conditions of both characters must have evolved multiple times throughout the evolutionary history of tree squirrels.

In the present paper, we describe Juliomys ossitenuis, a new species of sigmodontine rodent from the Altantic forest biodiversity hotspot in South America. This new species can be distinguished from the two congeners by clear morphological, molecular, and karyological characters. Juliomys ossitenuis is known from rain and semi-deciduous forests above 800 meters of altitude in southeastern Brazil, ranging from the state of Espírito Santo to São Paulo. Molecular phylogenetic analyses based on the mitochondrial cytochrome b gene confirmed that members of this new species form a well-supported monophyletic group, highly divergent from the other two species in the genus.

The Monodelphis brevicaudata complex comprises the short-tailed opossum M. brevicaudata and allied forms that inhabit northern and middle South America. We studied the systematics of this complex through a combination of molecular (cytochrome b and 16S rDNA) and morphological (external and craniodental) characters. Our evidence shows that M. brevicaudata as currently recognized comprises three different species: M. brevicaudata; Monodelphis touan, resurrected from the synonymy of M. brevicaudata; and a new species described here. Other species formally recognized in the M. brevicaudata complex include Monodelphis palliolata, Monodelphis glirina, Monodelphis maraxina, Monodelphis domestica, and two additional forms for which formal descriptions are still missing.

Members of the didelphid marsupial genus Monodelphis have previously been classified into species groups, but such informal subdivisions are inconsistent among authors and have contributed little but confusion to the taxonomic history of the genus. Recent molecular phylogenetic studies, however, have consistently recovered several strongly supported multispecies clades that merit formal recognition. Here we define and illustrate morphological characters that are useful for diagnosing these monophyletic groups, which we name and rank as subgenera to conserve binomial usage: Monodelphis Burnett, 1830 (for M. arlindoi, M. brevicaudata, M. domestica, M. glirina, M. palliolata, M. sanctaerosae, and M. touan), Microdelphys Burmeister, 1856 (for M. americana, M. gardneri, M. iheringi, and M. scalops), Monodelphiops Matschie, 1916 (for M. dimidiata and M. unistriata); Mygalodelphys, new subgenus (for M. adusta, M. handleyi, M. kunsi, M. osgoodi, M. peruviana, M. pinocchio, M. reigi, and M. ronaldi); and Pyrodelphys, new subgenus (for M. emiliae). We provide morphological diagnoses and describe pairwise comparisons to facilitate subgeneric identification, and we summarize subgeneric patterns of geographical distribution and sympatry to supplement recently published biogeographic analyses of the genus.

Aim To infer historical-biogeographical patterns and processes in South America based on estimated divergence times and ancestral distributions for a diverse clade of small marsupials (Monodelphis).Location South America.Methods We estimated a time-scaled phylogeny using sequence data from four nuclear genes and one mitochondrial gene for most currently recognized species, which we also scored for biome occupancy. We used maximum-likelihood methods that explicitly model historical-biogeographical processes to estimate ancestral distributions. ResultsThe earliest cladogenetic events in the genus occurred in the late Miocene, with subsequent diversification from the early Pliocene to the late Pleistocene. Model selection suggests an important role for founder-event speciation, a process seldom accounted for in previous biogeographical analyses of continental clades. Two equivalently optimal models that incorporate this process each reconstruct the most recent common ancestor of Monodelphis as occurring in both Amazonia and the Atlantic Forest, from which the Andes, the Arid Diagonal (Caatinga, Cerrado, Chaco) and other neighbouring biomes were colonized during the Pliocene and Pleistocene.Main Conclusions The diversification of Monodelphis was not restricted to a narrow time interval, suggesting that speciation cannot be attributed to a single leading factor such as tectonism or Pleistocene climatic fluctuations. In particular, speciation within the Atlantic Forest biome appears to have occurred in the Neogene, whereas most Amazonian speciation events are substantially younger. Amazonia and/or the Atlantic Forest hosted most diversification events within Monodelphis and were the historical sources of lineages that subsequently colonized other areas. Founder-event speciation, a concept traditionally associated with long-distance dispersal among islands, must sustain other interpretations in historical-biogeographical analyses of non-vagile organisms in continental contexts. Possible mechanisms for founder-event speciation in Monodelphis include historically transient connections between currently disjunct biomes and rapid reproductive isolation of populations colonizing adjacent but ecologically disparate biomes.

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