Friday, May 2, 2014

Do molecular data support an ancient rapid radiation of the genus Nicrophorus (Coleoptera: Silphidae)?

Do molecular data support an ancient rapid radiation of the genus Nicrophorus (Coleoptera: Silphidae)?
Chandra Venables & Derek S. Sikes

Department of Biological Sciences, University of Calgary, Calgary, AB, T2N 1N4, Canada




Abstract: Preliminary phylogenetic analyses of the genus Nicrophorus (Silphidae: Nicrophorinae) show a polytomy in the mid-level of the clade, which may reflect a rapid radiation. The resolution of the polytomy may be possible with the addition of data from genes evolving at a rate informative for the depth of the polytomy. This, combined with dense taxon sampling may 1) indicate whether the rapid radiation is ‘real,’ and 2) if the branching pattern of the polytomy can be resolved. Recent Bayesian analysis of molecular data indicates resolution may be possible. The latest trees suggest a number of intriguing macro-evolutionary scenarios and possible solutions to long-standing problems.



Table 1. Summary of molecular data analyzed, including the protein coding genes COI, COII (mitochondrial), and CAD (nuclear), and the non-prolein ooding nuclear D2 region of 2BS_ Parsimony informative characlers are for the in-group only (genus Nicrophorus).

Figure 1. 90% GTR+I+G Bayesian analysis of COII dala_Phylogeny shows a polytomy at the mid-level of the clade indicating a possible rapid radiation.


Introduction: The polytomy in the nicrophorine phylogeny (Fig. 1) may represent a rapid radiation (Sikes 2003). This radiation may have coincided with an EoceneI0|igocene faunal turn-over, when populations of large mammals were replaced by rodents (prey items of Nicrophorus) and lagomorphs in a radiation originating in Asia (Meng and McKenna 1998). The oldest nicrophorine fossils are from the Eocene (55-40 Mya) (Hatch 1927) thus, a radiation coincident with that of the rodents is possible. The species of Nicrophorus hypothesized to be basal to the rest of the genus are native to China. However, the lack of resolution at the mid-level of the nicrophorine tree (Fig.1) prevents a test of this hypothesis. Other information is obscured by the polytomy in the Nicmphoms phylogeny, such as: Where did the genus originate? Did the nicrophorines reach their present distribution (Fig. 2) via dispersal or vicariance? Do the New World species form a clade?

Methods: Our dataset contained 3,982 molecular characters (Table 1). 62 species were represented in the dataset including 6 outgroup species and 58 of the total 67 species in the genus Nicrophorus (B7%). Phylogenetic analyses were Bayesian (Bl) using MC’ algorithms. We conducted mixture model (BayesPhylogenies: Pagel and Meade [2004]), and single model analyses (MrBayes 3.1.22 Huelsenbeck and Ronquist [2001]). ln all analyses the best tit model of evolution was found to be GTR+I+G (ModelTest: Posada and Crandall [1998]). Two chain Bl analyses were run for 3x106 generations, sampling once every 1,000 generations. We made a 90% majority-rule consensus tree of 2,400 post burn-in trees. Mixture model analysis consisted of the GTR+G model with 2 rate-matrices and 2x106 generation MC3, sampling once every 1,000 generations, resulting in a post burn-in total of 1,500 trees.



Discussion/Conclusions: Single and mixture model Bayesian analyses of sequences from 4 genes with dense taxon sampling support the hypothesis that the genus Nicrophorus has experienced a rapid radiation (Fig. 3 & 4). Resolution of the branching pattem at the mid-level of the tree suggest the following:

- The basal-most species of Nicrophorus are N. smefarka and N. przewalskii (Fig. 4 & 5). These species are both endemic to China which suggests that the ancestral Nicmphorus may have occurred in China.

- Given the probable origin of the lineage in the Eocene (Sikes 2003), and the location of the ancestral species, it is possible that the nicrophorine radiation coincided with the rodentl lagomorph radiation in Asia (Meng and McKenna 1998).

- These results suggest Nicrophorus invaded the New World a minimum of 3 and a maximum of 10 times. Resolution of the polytomy in the terminal clades, and complete taxon sampling should resolve the exact number, and pattem of invasions.

- The monophyly of the South American species indicates a single invasion of South America (Fig. 3). However, not all S. American species are represented, so it is possible that multiple invasions have occurred.

Acknowledgements: l‘d like to thank my colleague Tonya Mousseau for her ideas and her encouragement. Funding was provided by NSERC awarded tc Derek Sikes.

Literature Cited:_ Hatch, ul. l-l. 1927. Studies cn the Silphinae. Journal ofthe New York Entomological Society 35: 331-370. l-luelsenbeelt, .l.P. s. l=. Ronquist. 2001. MRBAYES: Bayesian inference of phylogeny. Bioinformatics 17: 754-755. Meng, J. & McKenna, M.C. 1995.
Faunal tumcvers of Palaeogene mammals from the Mongolian Plateau. Nature 394: 364-367. Pagel, ul., and A. Meade. 2004. A phylogenetic mixture model for detecting pattern heterogeneity in gene sequence or character state data. Systematic Biclcgy 53(4): 571-581.
Posada, o. and Crandall, K.A. 1995. Modeltest: testing the model of DNA substitution. Bioinformatics 14:817-s1a. sikes, o.s. zoos. A revision ofthe subfamily Nisroohorinae lGrby (lnseotacoleootera;silpnidae) PhD Dissertation. University of Connecticut, Connecticut, us.


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