GENETIC DIFFERENTIATION IN MEXICAN CONIFERS
F. Thomas Ledig1, Basilio Bermejo-Velázquez2, and Jesús Vargas-Hernández3
1 Institute of Forest Genetics, USDA Forest Service, Placerville, California, U.S.A.
2 Centro de Genetica Forestal, Universidad Autonoma Chapingo, Chapingo, México, MÉXICO
3 Programa Forestal, Colegio de Postgraduados Montecillo, México, MÉXICO
ABSTRACT
Most of the genetic diversity in molecular markers of conifers of boreal
and north temperate North America, Europe, and Asia is present within
populations, and differences among populations constitute a small
percentage of the total.
We reviewed the emerging literature on Mexican conifers to determine
whether this generalization can be extended to them as well. All of our
data refer to isozyme diversity, and the taxa include spruces, firs, and
pines.
Levels of differentiation were two to three times higher in Mexican
conifers. México’s extremes in topography and associated climate mean
that many species occur as insular populations, they exist in pockets of
favorable habitat separated by many kilometers, often of arid desert.
Migration among pockets is restricted, and populations are either small
or have experienced bottlenecks as their habitat shrunk during climate
change. This favors genetic drift or differentiation by selection. Drift
and selection in isolation can lead to speciation, which may explain why
México is a secondary center of diversity for pines.
INTRODUCTION
When the use of isozyme electrophoresis became common practice, data
on the genetic structure of plant populations increased rapidly. Conifers,
in particular, have been well-studied because their haploid
megagametophyte persists until seed maturity, which makes genetic
analysis simple. Hamrick and Godt (1996) counted 274 studies in the
Pinaceae alone. Most of the early studies on the genetic structure of
conifers dealt with widespread, north temperate or boreal species.
One of the emerging generalities was that conifers had substantial
genetic diversity within populations and only low levels of
differentiation among populations. The common measure of
differentiation was GST or FST, which yielded nearly identical values.
GST (or FST) is the proportion of the total genetic diversity within a
species that is attributed to differences among populations. For 28 north
temperate pines, GST averaged 0.076 (Ledig 1998). In a review of
genetic structure in eight species of boreal and north temperate spruce
species, GST averaged 0.047 (Ledig et al. 1997). Thus, on average, over
92% of the genetic diversity within pine species is within populations,
and in spruces, over 95%.
Genetic structure has implications for conservation. If nearly all of the
genetic variation is within populations, then reserves that protect a few,
or even a single, large population will be sufficient to conserve most of
the diversity.
MEXICO - AN EXCEPTION
Evidence beginning to accumulate suggests that the generalities deduced
from studies of the north temperate and boreal tree species of North
America, Europe, and Asia may not apply in Mexico and Central
America. For example, Francisco Molina-Freaner and his collaborators
(Molina et al. 1999) recognized that weeping piñon (Pinus pinceana G.
Gordon & Glendinning), Laguna piñon (P. cembroides Zuccarini subsp.
lagunae (Robert-Passini) D. K. Bailey), and bishop pine (P. muricata D.
Don) had higher levels of diversity than commonly encountered in more
northern species or populations of pines. They realized that
conservation of these species in Mexico would be effective only if
several populations were protected.
OBJECTIVES
We reviewed our data on Mexican conifers, most of it unpublished, and
the published results of others to determine whether population
differentiation in Mexico was greater than that observed among north
temperate and boreal conifers.
METHODS
Our studies of Mexican conifers included four pines and three spruces.
The pines were Mexican white pine (Pinus ayacahuite Ehrenberg ex
Schlectendal), Apache pine (P. engelmannii Carriére), Gregg’s pine (P.
greggii Engelmann), and weeping piñon (Pinus pinceana G. Gordon &
Glendinning). The spruce were Chihuahua spruce (Picea chihuahuana
Martínez), Martínez spruce (P. martínezii T. F. Patterson), and Mexican
spruce (P. mexicana Martínez). For each species, isozyme
electrophoresis provided genetic markers. Using BIOSYS (Swofford
and Selander 1981), we calculated FST from the allele frequencies at
several loci. Because each species was the object of a separate study, the
number of loci and the number of populations varied. The fewest
number of loci was 13 in Gregg’s pine and the largest was 27 in weeping
piñon. The number of populations sampled varied from 2 in Martínez
spruce up to 23 in Apache pine.
Chihuahua spruce (Picea chihuahuana) at El Realito
Mexican weeping piñon (Pinus pinceana) in Querėtaro
RESULTS
FST varied from 0.024 to 0.337 in the seven species of pines and spruces
we studied (Table 1).
Table 1. FST in seven species of pines and spruces.
Pop.1
Loci 2
10
24
FST
0.248
Picea martínezii
2
22
0.024
Picea mexicana
3
17
0.079
Pinus ayacahuite
14
23
0.222
Pinus engelmannii
23
26
0.130
Pinus greggii
10
16
0.337
8
27
0.152
Species
Picea chihuahuana
Pinus pinceana
Mean
1Number of populations sampled
0.170
2Number of isozyme loci surveyed
Mexican spruce (Picea mexicana) on Sierra La Marta
DISCUSSION
Our data and that of others (Table 2) suggests that differentiation among
populations within conifers is more pronounced in Mexico and Central
America than in northern latitudes. In our studies of seven species of
pines and spruces, diversity among populations averaged 17% of the
total. In ten other studies of Mexican and Central American species, the
average was 17.2% of the total diversity. These values are compared to
7.6% among populations of 28 pines, 4.7% among populations of eight
spruces, and 3.4% among populations of five firs native to north
temperate and boreal latitudes (Table 2). Based on standard errors of the
means, the differences between Mexican conifers and those of the north
are real.
Levels of diversity in Mexican and Central American conifers are
generally as high as those in species from northern temperate latitudes,
but levels of differentiation may be twice as great in Mexico. The most
obvious reason for higher levels of differentiation in Mexican and
Central American species is that they are more highly fragmented than
those of northern latitudes. Because of México’s extremes in
topography and associated climate, many species occur as insular
populations. In Mexico, both tropical jungle and arid desert may be
separated from alpine habitat by less than 50 kilometers (McDonald
1993). Conifers are primarily montane species, and populations exist in
pockets of favorable habitat separated by many kilometers. Migration
among pockets is probably restricted, and populations are either small or
have experienced bottlenecks as their habitat shrunk during climate
change. This favors genetic drift or differentiation by selection. Drift
and selection in isolation can lead to speciation, which may explain why
México is a secondary center of diversity for pines.
North temperate pines that have highly fragmented ranges, like their
congeners in Mexico, also are characterized by high levels of
differentiation. For example, populations of Coulter pine (Pinus coulteri
D. Don) in California may be separated by gaps of up to 120 km, and
FST for the species is 0.165, similar to the average for Mexican and
Central American pines (Ledig 2000).
Many conifers of northern latitudes expanded into their present range
within the last 10,000 years, leaving little time for differentiation to
occur. The ancestors of the present-day jack pine (Pinus banksiana
Lambert) and lodgepole pine (Pinus contorta Douglas ex Loudon), for
example, were in contact in the geologically recent past. The lack of
differentiation within these species (FST of 0.039 and 0.034,
respectively, averaged across studies tabulated in Ledig 1998) reflects
recent gene flow.
Mexican white pine (Pinus ayacahuite) on Cerro Potosi
Table 2. Review of genetic diversity among populations of Mexican and
Central American conifers.
Species
Pop.1
Loci 2
Reference
16
FST 3
0.271
Abies flinckii
6
Abies guatemalensis
10
16
0.122
Aguirre-P. et al. (2000)
Abies hickelii
6
16
0.073
Aguirre-P. et al. (2000)
Abies religiosa
11
16
0.250
Aguirre-P. et al. (2000)
Pinus caribaea
4
16
0.131
Matheson et al. (1989)
Pinus lagunae
4
14
0.188
Molina-F. et al. (1999)
Pinus muricata
3
13
0.161
Molina-F. et al. (1999)
Pinus oocarpa
5
16
0.104
Matheson et al. (1989)
Pinus pinceana
5
13
0.247
Molina-F. et al. (1999)
Pinus rzedowski
9
14
0.175
Delgado-P. et al. (1999)
Mean
0.172(0.067)4
Aguirre-P. et al. (2000)
Current Study
0.170(0.107)
Northern firs
0.034(0.027)
Various authors5
Northern pines
0.076(0.068)
Ledig 1998
Northern spruces
0.047(0.036)
Ledig et al. 1997
1Number of populations sampled
2Number of isozyme loci surveyed
3Or G in some cases.
ST
4Standard deviation calculated among species (in parentheses).
5Mean of: Abies alba Mill. from Breitenbach-Dorfer et al. (1997); A. cephalonica Loudon from
Fady and Conkle (1993); A. fraseri (Pursh) Poir. from Diebel and Feret (1991); A. koreana
Wilson from Chung and Lee (1985); A. lasiocarpa (Hook.) Nutt. from Shea (1990).
CONCLUSION
Fragmentation and reduced gene flow in the rugged topography of
Mexico and Central America have apparently led to high levels of
differentiation among populations of several, if not all, native conifers.
Either selection in diverse environments or genetic drift in small,
isolated populations may have led to differentiation. Genetic drift is the
most likely explanation in some species (e.g., Ledig et al. 1997).
The implications of such high levels of genetic structure have already
been suggested by Molina-F. et al. (1999): it will be necessary to
conserve several or many populations of each species to preserve their
genetic diversity.
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