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Systematics Section / ASPT

Gernandt, David S. [3], Magallón, Susana [2], Geada López, Gretel [4], Liston, Aaron [1].

Penalized likelihood reveals inconsistencies between divergence time estimates and fossil assignments in Pinus and Pinaceae.

The fossil record of Pinaceae extends to the Cretaceous for Pinus, Picea, Pityostrobus, and Pseudoaraucaria and to the late Jurassic for Pseudolarix. We used these fossils together with several late Cretaceous and Tertiary fossils assigned to infrageneric taxa of Pinus to estimate minimum divergence times in Pinaceae. Chloroplast sequences of rbcL and matK for Pinaceae, with particularly dense taxon sampling in Pinus, were used together with an Araucaria outgroup to estimate a phylogeny using Bayesian inference. Maximum likelihood branch lengths for this phylogeny were analyzed with penalized likelihood in r8s. When the early Cretaceous taxon Pinus belgica was used as a calibration point, divergence times were close to previous molecular clock estimates from matK. We then constrained the minimum age of several other nodes using 1) published assignments of fossils to infrageneric taxa in Pinus and 2) older age estimates for fossils thought to represent other extant genera in Pinaceae. Several of these subsequent calibrations yielded Pinaceae divergence times that dramatically predated maximum ages hypothesized for seed plants. A prime example of this inconsistency was observed when the minimum age for the divergence of Pinus subgenus Strobus was constrained to 83 mya by accepting the assignment of late Cretaceous fossil wood (Pinuxylon sp.) to section Parrya and a cone (P. magothensis) to section Quinquefolius. This result may have been caused by a low value for the smoothing parameter used in penalized likelihood and does not necessarily reflect the need for a more conservative interpretation of these fossils. We also examined the sensitivity of divergence estimates by excluding species with elevated rates of substitution.

1 - Oregon State University, Department of Botany and Plant Pathology, 2082 Cordley Hall, Corvallis, Oregon, 97331-2902, USA
2 - Universidad Nacional Autónoma de México, Department of Botany, Circuito Exterior, Anexo al Jardín Botánico, A.P. 70-233, Mexico City, 04510, Mexico
3 - Universidad Autonoma del Estado de Hidalgo, Centro de Investigaciones Biologicas, AP 1-69, Plaza Juarez, Pachuca, Hidalgo, 42001, Mexico
4 - Universidad de Pinar del Rio, Departamento de Biologia, Marti 270 final, esq. a 27 de Noviembre, Pinar del Rio, , Cuba


Presentation Type: Paper
Session: 45-8
Location: Cottonwood B (Snowbird Center)
Date: Wednesday, August 4th, 2004
Time: 10:00 AM
Abstract ID:727

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