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2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 11<br />

olis-coupled Markov-chain Monte Carlo<br />

(MCMCMC), each with one cold <strong>and</strong> three<br />

incrementally heated chains. For each run, we<br />

assumed uniform-interval priors for all parameters,<br />

except base composition, which<br />

assumed a Dirichlet prior. Runs were allowed<br />

to proceed for 5 3 10 6 generations, <strong>and</strong> trees<br />

were sampled every 100 generations. We<br />

evaluated the burn-in for each run, <strong>and</strong><br />

pooled the post-burn-in trees to calculate<br />

estimated parameter distributions <strong>and</strong> posterior<br />

probabilities for each node. We assessed<br />

nodal support from MP <strong>and</strong> ML analyses<br />

using nonparametric bootstrapping (Felsenstein,<br />

1985). Bootstrap values for the parsimony<br />

analyses (MPBS) were calculated in<br />

PAUP* from 1000 pseudoreplicated datasets,<br />

each <strong>of</strong> which was analyzed heuristically with<br />

10 r<strong>and</strong>om-addition replicates with TBR<br />

branch swapping. Bootstrap values for the<br />

likelihood analysis (MLBS) were calculated in<br />

GARLI using genetic-algorithm searches <strong>of</strong><br />

1000 pseudoreplicated datasets, allowing<br />

model parameters to be estimated for each<br />

pseudoreplicate.<br />

We analyzed the combined-gene dataset<br />

using ML as implemented in RAxML-VI-<br />

HPC (ver. 2.2.3; Stamatakis, 2006). We<br />

specified the GTRMIX model, which performs<br />

initial tree inference using a GTRCAT<br />

approximation, with final topology evaluation<br />

performed under a GTRGAMMA<br />

model. We allowed parameters to be estimated<br />

independently across the five gene partitions.<br />

To evaluate nodal support for this<br />

combined-gene, mixed-model analysis, we<br />

performed 1000 bootstrap replicates, again<br />

allowing model parameters to be estimated<br />

independently across the five genes. We also<br />

performed a Bayesian analysis <strong>of</strong> the combined-gene<br />

dataset, using the same MCMCMC<br />

settings given above. For this analysis, we<br />

specified the best-fit model for each gene,<br />

decoupled estimation <strong>of</strong> substitution parameters<br />

across the partitions, <strong>and</strong> allowed each<br />

gene to assume a separate rate.<br />

We analyzed the nonmolecular (morphological<br />

+ karyotypic) data alone <strong>and</strong> in<br />

combination with the molecular data using<br />

MP <strong>and</strong> Bayesian approaches. Due to the<br />

large number <strong>of</strong> suboptimal trees recovered<br />

from parsimony analysis <strong>of</strong> the nonmolecular<br />

dataset, we first performed 1000 r<strong>and</strong>om-<br />

taxon-addition replicates with TBR branch<br />

swapping, but saved only 10 trees per<br />

replicate. We then used this pool <strong>of</strong> 10,000<br />

trees as the starting point for an unbounded<br />

heuristic search. Parsimony analysis <strong>of</strong> the<br />

combined (molecular + nonmolecular) dataset<br />

did not exhibit this problem; therefore, we<br />

analyzed the combined dataset using unbounded<br />

heuristic searches with 1000 replicates<br />

<strong>of</strong> r<strong>and</strong>om-taxon addition <strong>and</strong> TBR<br />

branch swapping. For Bayesian analysis <strong>of</strong><br />

the morphological dataset alone, we specified<br />

the Mkv model (Lewis, 2001) with a Cdistributed<br />

rate parameter <strong>and</strong> ascertainment<br />

bias corrected for omission <strong>of</strong> constant characters<br />

(lset coding 5 variable). For Bayesian<br />

analysis <strong>of</strong> the combined (molecular + nonmolecular)<br />

dataset, we specified this same<br />

model for the morphological data <strong>and</strong> applied<br />

the best-fit model to each <strong>of</strong> the five gene<br />

partitions. As above, we allowed parameters to<br />

be estimated independently across all partitions<br />

<strong>and</strong> used the same MCMCMC settings.<br />

Toassesstheimpact<strong>of</strong>thelargeamount<strong>of</strong><br />

missing data from Chacodelphys, we performed<br />

all analyses that included nonmolecular<br />

characters with <strong>and</strong> without this taxon.<br />

ONLINE DATA ARCHIVES: All <strong>of</strong> the new<br />

molecular sequences produced for this study<br />

have been deposited in GenBank with<br />

accession numbers FJ159278–FJ159314 <strong>and</strong><br />

FJ 159316–FJ159370 (for a complete list <strong>of</strong><br />

GenBank accession numbers <strong>of</strong> all analyzed<br />

sequences, old <strong>and</strong> new, see table 9). All <strong>of</strong><br />

our datasets, selected ML <strong>and</strong> Bayesian<br />

analyses, <strong>and</strong> associated trees have been<br />

deposited on TreeBase (http://www.treebase.<br />

org) with accession numbers S2164, M4107,<br />

<strong>and</strong> M4108. Our nonmolecular data matrix<br />

has also been deposited on MorphoBank<br />

(http://morphobank.geongrid.org) with accession<br />

number X600.<br />

COMPARATIVE MORPHOLOGY<br />

The literature on <strong>didelphid</strong> comparative<br />

morphology is widely scattered, <strong>and</strong> no<br />

adequate review <strong>of</strong> this topic has yet been<br />

published. Although the following accounts<br />

are far from comprehensive, they include<br />

most <strong>of</strong> the anatomical features that have<br />

been surveyed widely among extant genera<br />

<strong>and</strong> that provide relevant taxonomic infor-

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