Population Genetics of Tropical
Tropical tree breeding patterns
We use polymorphic allozyme and microsatellite marker loci to examine
breeding patterns within populations of several tropical tree species.
Our research indicates that the majority of the species analyzed (about
20 species to date) are outcrossing. We have also shown that the density
of flowering individuals has a distinct impact on the genetic composition
of pollen received by each tree; species with low densities of flowering
individuals have greater heterogeneity in pollen allele frequencies than
species that occur and flower at higher densities. By using multi-locus
analyses of large progeny arrays (i.e., paternity analyses) several breeding
structure parameters can be determined; the proportion of long-distance
gene movement, pollen movement patterns within populations, and the relative
success of individuals as pollen donors. Our results indicate that male
reproductive success varies greatly among individuals. Pollen movement
in most of the species examined is extensive with 50% or more of the pollen
traveling 500 m or more.
We have also taken advantage of the unique breeding systems of figs (Ficus),
mimosoid legumes and orchids (all produce fruits in which the seeds are
full-sibs i.e., there is a single pollen donor) to estimate the number
of individuals contributing pollen to an individual tree. We have found
that the number and identity of the pollen donors varies greatly among
maternal plants and among years for individual plants. This full-sib
paternity analysis provides a level of detail of the breeding system that
is unavailable for any other analyses. We are continuing these analyses
to better understand spatial and temporal variation in the breeding patterns
of these species.
forest fragmentation As has been well
documented, tropical forests are being destroyed at an alarming rate.
Such landscape level destruction often produces fragments of the original
forest that vary considerably in their size and in the degree of physical
isolation. Much of my current research is focused on the genetic consequences
of tropical forest fragmentation. The majority of this research occurs
in the tropical dry forest of Guanacaste Province in Costa Rica. The chief
study species is Enterolobium cyclocarpum,
the Guanacaste tree. Enterolobium cyclocarpum
is an insect-pollinated mimosoid legume which is a dominant tree in the
drier regions of Central America. We use full-sib analyses to compare
rates of pollen movement in heavily disturbed and relatively undisturbed
We are also investigating the role that isolated
individuals play in the breeding populations of this species. Our preliminary
results show that, on average, 75% of the effective pollen is the result
of long-distance (>500m) dispersal by its nocturnal insect pollinators.
Furthermore, it appears that trees in isolated fragments receive pollen
from as many or perhaps more pollen donors than trees in undisturbed forest.
Isolated trees, somewhat surprisingly, receive pollen from a greater number
of pollen donors than trees that grow in less isolated situations indicating
that these isolated trees may play a significant role in the breeding
populations of this species. Recently we have begun to investigate how
the flowering phenology of individual trees is associated with the patterns
of mating we have documented genetically.
Dr. Peter Smouse (Rutgers University) and I are applying
an analytical procedure termed Two-Gener to analyze the breeding system
and gene flow in several E. cyclocarpum populations. The Two-Gener
approach, which was developed by Dr. Smouse, allows one to obtain estimates
of the effective number of pollen parents and pollen movement distances
without performing a complete paternity analysis. This approach allows
us to partition the data to examine the effects of habitat type, distance
to the nearest neighbor, and individual tree phenology on these breeding
structure parameters. These results are being compared directly to those
from the full-sib analyses discussed above. The Two-Gener analysis will
also be applied to an expanded sampling scheme of A. julibrissin
in the Athens, Georgia area. Seed collections are being made from 50+
clusters of three trees each. This will allow us to determine how much
of the tree-to-tree variation in the pollen donor pool occurs at a local
scale (<50m) or whether most of the variation in the pollen donor pool
occurs at a larger spatial scale (> 1km).
Aldrich, P. R. and J. L. Hamrick. 1998. Reproductive
dominance of pasture trees in a fragmented tropical forest mosaic. Science
Apsit, V. J., J. L. Hamrick and J. D. Nason. 2001. Breeding population
size of a fragmented population of a Costa Rican dry forest tree species.
Journal of Heredity, 92:415-420.
Murawski, D. A. and J. L. Hamrick. 1991. The effect of the density of
flowering individuals on the mating systems of nine tropical tree species.
Hufford, K. M. and J. L. Hamrick. 2003. Viability selection at three early
life stages of the tropical tree, Platypodium elegans (Fabaceae,
Papilionoideae). Evolution 57:518-526.