GEOGRAPHICAL GRADIENT OF CHROMOSOMAL

Rev. Int. Contam. Ambient. 21 (Supl. 1) 21-25, 2005
GEOGRAPHICAL GRADIENT OF CHROMOSOMAL POLYMORPHISM IN MEXICAN
POPULATIONS OF Drosophila pseudoobscura
Judith GUZMÁN RINCÓN1, 2, Víctor Manuel SALCEDA1, Olga OLVERA R.1 and Louis LEVINE3
Depto. de Biología, ININ. Km 36.5 Carretera México-Toluca, México. E-mail: judisabel@yahoo.com.mx
Laboratorio de Mutagénesis Ambiental, Centro de Ciencias de la Atmósfera, UNAM, México
3
Department of Biology, City College of New York, Convent Avenue at 138th Street, New York, N.Y. 10031,
USA
1
2
Keywords: D. pseudoobscura, chromosomal polymorphism, evolution
ABSTRACT
In order to better understand the evolution of Mexican populations of D. pseudoobscura,
the frequencies of the chromosomal arrangements of flies collected at Valparaíso, Zacatecas,
La Congoja, Aguascalientes, and Los Reyes and Río Verde, San Luis Potosi, were analyzed.
TL was the ubiquitous arrangement and was found in all the sampled populations. Three
out of the four populations are very polymorphic, showing either 10 or 11 different
arrangements; on the other hand, Los Reyes showed only 5. The most frequent arrangements
were Santa Cruz (SC) and Tree Line (TL), whose added value is between 65 and 79 % of the
total. The frequencies of these arrangements showed an inverse relationship: whereas SC
was abundant in the West (54 %) and decreased in the East (14 %), TL was scarce in the
West (32 %) and increased in the East (52 %). The behavior of Olympic (OL) was similar to
TL, but with lower frequencies (1 – 16 %). Cuernavaca (CU) and Estes Park (EP), on the
other hand, showed extremely low frequencies.
Palabras clave: D. pseudoobscura, polimorfismo cromosómico, evolución
RESUMEN
Para entender mejor el comportamiento de las poblaciones mexicanas de D.
pesudoobscura se analizaron las frecuencias de los arreglos cromosómicos de moscas
colectadas en Valparaíso, Zacatecas, La Congoja, Aguascalientes, así como en Los
Reyes y Río Verde, San Luis Potosi. El arreglo Tree Line (TL) estuvo presente en todas
las poblaciones estudiadas, además 3 de las 4 poblaciones fueron muy polimórficas con
11 y 10 arreglos diferentes, aunque Los Reyes presentó sólo 5 arreglos. Los más
frecuentes fueron Santa Cruz (SC) y TL, cuya suma fluctuó entre 65 y 79 % y presentó
una relación inversa, SC es abundante en el oeste (54 %) y disminuye en el este (14 %),
TL es escaso en el oeste (32 %) y se incrementa en el este (52 %). Olimpic (OL) fue
semejante a TL, pero con frecuencias bajas (1 a 16 %), mientras que Cuernavaca (CU) y
Estes Park (EP) presentaron frecuencias extremadamente bajas.
INTRODUCTION
Population genetics studies the effects of Mendelian
Laws and other genetic principles on groups of individuals. Its analyses are fundamental to the understand-
ing of evolution, as they reflect the progressive change
in the genetic composition of populations. Population
genetics also permits us to understand the effects of
environmental factors on natural populations as well
as the complex phenomenon of evolution.
22
J. Guzmán Rincón et al.
Drosophila pseudoobscura is a species which
has contributed to the substantial advance of population genetics. The species shows a geographical distribution which goes from the British Columbia in
Canada, through all of the western USA, to México
and Guatemala, and an isolated population occurs near
Bogotá, Colombia (Dobzhansky et al. 1963). This fly
inhabits cool weathered places, mainly pine-holm oak
mesophilic forests. It is not found in Central America,
probably because of climatic and geographic reasons,
but it reappears again in the high Colombian Plateau
as an isolated population. Its specific feeding requirements are unknown, but they are easily grown in the
laboratory. It has polytenic chromosomes which are
the result of successive endomitoses and are formed
by 1024 bounded chromosomes.
Shortly after Painter demonstrated that giant chromosomes of the salivary gland of the larvae are useful tools in the study of the genetics of Drosophila,
Sturtevant and Dobzhansky (1936) proved that these
chromosomes contain multiple inversions which are
maintained within the populations. It was first thought
that inversions were neutral variants, and irrelevant
for evolutionary adaptation. However, it was soon
discovered that these polymorphisms are exposed to
very strong selective pressure which allowed the study
of the dynamics of the evolutionary process in a reasonable period of time, thereby turning the experimental manipulation and observation of the chromosomes into a profitable effort. Since Darwin, it has
been generally assumed that inheritable variation,
from which adaptations are modelled, is under a weak
selective pressure; hence it is virtually hopeless for
the biologists to study its dynamics within one generation. The inversions of Drosophila are a very
promising exception.
There are a great number of species of Drosophila which are polymorphic for chromosomal inversions. Dobzhansky and Sturtevant (1938) demonstrated that natural populations of D. pseudoobscura
showed chromosomal inversions which were naturally maintained, forming chromosomal polymorphisms. It was evident that the third chromosome of
D. pseudoobscura had an extraordinary diversity of
chromosomal arrangements. Inversions are very rare
events thus, a particular inversion is presumed to have
occurred only once within a phylad. Because of this, it
is generally agreed that chromosomal arrangements
carry more historical information than mutations (Olvera
et al. 1979), hence they could be used to make phylogenetic trees which reflect the evolutionary history of
the species (Fig. 1).
Dobzhansky and Epling (1944) also noted that, with
some exceptions, the difference in the genetic composition of any two populations rises as the distance
separating them increases, the gradients may some
times be sharp and at other times smooth. A number
of studies have been reported dealing with the geographic gradients of inversions in populations found
in the border between México and the USA, as well
as in Central México (Guzmán et al. 1993).
The aim of this study was to get a better understanding of the phenomenon involved in the geographic
variation and to map the distribution of the polymorphisms in Mexican populations, as well as their role
in the evolution of the species. In order to achieve
this, the chromosomal frequencies of populations in
an East-West transect between parallels 21 and 22°N
were analyzed.
MATERIALS AND METHODS
Flies were collected in Valparaíso, Zacatecas; La
Congoja, Aguascalientes; and Los Reyes and Río
Verde, San Luis Potosí. Fermented banana was used
to attract the flies, which were classified at the collection site. Individuals of the pseudoobscura group
were brought to the laboratory, where each female
was placed in a separate culture bottle, while the males
were individually crossed with Estes Park (EP) females from the laboratory. Cultures were incubated
at 24°C and 75 % RH. One or more third instar larvae were dissected, and their salivary glands removed
and stained with aceto-lactic orcein. Squash preparations were made of the glands and their chromosomes studied. The inversion types were classified
according to the descriptions in Dobzhansky and
Sturtevant (1938), Kastritsis and Crumpacker (1966,
1967) and Olvera et al. (1979, 1985).
RESULTS AND DISCUSSION
The number of chromosomes analyzed varied from
542 for Río Verde, San Luis Potosí, to 73 in Los Reyes,
also in San Luís Potosí (Table I). All populations
showed a great deal of polymorphism: 11 arrangements for Valparaíso, 11 for La Congoja, 10 for Río
Verde, and 5 for Los Reyes.
The main arrangements found in the four localities
were SC and TL; SC being the most frequent for populations of Valparaíso (42 %) and La Congoja (43 %),
and TL for the populations of San Luis Potosí (50 and
52 % for Río Verde and Los Reyes respectively)
(Table I).
Geographic distribution patterns, called cline,
show gradual changes in the frequencies of the arrangements which follow a direction in the adjacent
populations.
23
GEOGRAPHICAL GRADIENTES OF POLYMORPHISM
OAXACA
(M)(M)
OAXACA
ESTES
PARK
(M) (M)
ESTES
PARK
MICHOACÁN
(M) (M)
MICHOACAN
TARASCO
(M) (M)TARASCO
OZUMBA
(M) (M)OZUMBA
LIRIOS
(M) (M)
LOS LOS
LIRIOS
Drosophila
miranda
Drosophila
miranda
(M)
(M)
(M)
(M)
HIDALGO
HIDALGO
ZIRAHUÉN
(M) (M)
ZIRAHUÉN
(M) (M)
AMECAMECA
AMECAMECA
PAXTEPEC
(M) (M)
PAXTEPEC
(M) (M)
MIRAFLORES
MIRAFLORES
TREE LINES
TREE
LINE
(M)
CUERNAVACA
(M)
(M)
(M)
CUERNAVACA
(M)
SAN
ANTONIO
SAN
ANTONIO
(M)
(M)
MAMOTH
(M)
MAMOTH
IZTACCÍHUATL
IZTACCÍHUATL
(M) TZINTZUNTZAN
TZINTZUNTZAN
URUAPAN
(M) (M)URUAPAN
BERKLEY
BERKLEY
(M)
SANTA
BARBARA
SANTA
BARBARA
(M)
CHIRICAHUA
CHIRICAHUA
(M)
SANTA
CRUZ
SANTA CRUZ
(M)
(M)
HYPOTHETICAL
HYPOTHETICAL
SAN
JACINTO
SAN
JACINTO
(M)
THOMAS
THOMAS
VANDEVENTER
VANDEVENTER
EAST
EASTBAY
BAY
STANDARD
STANDARD
TEXAS
TEXAS
PIKES
PEAK(M)(M)
PIKES
PEAK
AMERICAN FORK
AMERICAN
FORK
FORT
COLLINS
FORT
COLLINS
SONOITA
Drosophila
persimilis
Drosophila
persimilis
SONOITA
ARROW
HEAD
ARROW HEAD
(M)
(M)
PINON
PINON
COCHISE
COCHISE
(M)
(M) (M)
OLYMPIC
OLYMPIC
(M) (M)
TULANCINGO
TULANCINGO
(M) (M)
POPOCATÉPETL
POPOCATÉPETL
Fig. 1. Phylogenia of the genetic arrangements of the third chromosome of Drosophila pseudoobscura in 1985 (Taken from Olvera et
al. 1985) (M) indicates the arrangements found in México
These changes are evident in D. pseudoobscura
if an East-West pattern is studied (Powell 1992).
When comparing our results with respect to those
reported previously (Dobzhansky and Epling 1944;
Guzmán et al. 1993) it is observed that clinal variation
among the populations found between parallels 21 and
22°N is very similar to that reported for parallels 30°N
and 20°N, nevertheless, the arrangements involved are
very different (Fig. 2). While in the North (30°N)
Arrow Head (AR) is the most frequent arrangement
in New Mexico (69.1 %), and decreases to very low
frequencies in the East (3.3 % in Valley area, Tex.), in
the Mexican populations SC is the most abundant
arrangement in the West, with frequencies of 42 %
TABLE I. PERCENTAGE OF GENETIC ARRANGEMENTS FOUND
IN THE THIRD CHROMOSOME OF D. Pseudoobscura IN
MEXICAN POPULATIONS LOCATED BETWEEN THE
PARALELS 21-22°N
Populations
Valparaíso
La Congoja
Los Reyes
Río Verde
Arragement
SC
TL
CU
OL
Other
Total
42
43
22
14
32
36
52
50
3
3
7
6
1
5
16
15
22
13
3
15
474
240
73
542
24
J. Guzmán Rincón et al.
5.- Guzmán
Cd. Guzmán (8)
(8)
5. Ciudad
6.- Zirahuén, Michoacán (11)
6. Zirahuén, Michoacán (11)
7.- Milpa Alta, D.F. (7)
7. Milpa 8.Alta,
D.F. (7)
El Seco, Puebla (7)
8. El Seco,
Puebla
(7) (5)
9.- Orizaba,
Veracruz
9. Orizaba, Veracruz (5)
Valparaiso (11)(11)
1.1.-Valparaíso
Congoja (11)
2.2.LaLaCongoja
(11)
3.- Los Reyes (5)
3. Los Reyes (5)
4.- Río Verde (10)
10.- Condados Doña Ana y
4. Río Verde (10)
Lincoln,
(5)
10. Doña
AnaNuevo
and México
Lincoln,
11.- Área
TransMéxico
– Pecos, Texas
Countys
New
(5)
.
(6)
12.- Texas sur-central (6)
11. Trans
- Pecos Area, Texas (6)
13.- Área del Valle, Texas (4)
12. South Central Texas (6)
13. Valley area, Texas (4)
0.6
%
0.6 0
0.45
40
0.3
20
0.15
80
%0
0.6
60
0.45
40
0.3
0
0 . 15
20
Santa Cruz
Santa Cruz
40
20
0
60
40
20
0
%
60
40
20
0
%
60
0
0.6
0%
80
0.45
60
0.3
40
0.15
20
0
0
10
10
1
11
11
2
12
12
3
13
13
4
11
22
33
44
40
Estes
Estes
Park
Park
Standard
Lines
Standar TreeTree
Line
Tree
TreeLines
Line
60
Olympic
Olympic
PikesPikes
Peak
Peak
80 0
%
60
%
Cuernavaca
0.15
Cuernavaca
20
Tree Lines
0.3
Tree Line
40
Santa
Santa Cruz
Cruz
0.45
60
Cuernavaca
Cuernavaca
Arrow
Head
Arrow
Head
80
%
15
20
10
10
5
0
0
5
55
6
7
8
66
77
88
9
99
Fig. 2. Distribution of the main arrangements of the populations located between parallels 20, 21-22 and 30°N (partial data,
taken from Guzmán et al. 1993)
in Valparaíso (between 21 – 22°N) and 65.7 % in
Ciudad Guzmán (between 19-20°N). On the other
hand, the inverse cline is presented by Pikes Peak (PP)
in the north, with a frequency of 24.2 % in New Mexico,
and becoming more important to the East like in Texas
populations with 67.7 %, while in populations between
parallels 21 and 22°N the complementary arrangement
is TL, with frequencies of 32 % in Valparaíso (East)
and of 52 % in Río Verde (West). In the populations of
parallel 20°N, CU plays an important complementary
role, showing frequencies which go from 5.3 % in
Ciudad Guzmán (West) to 51.8 % in Orizaba (East)
(Dobzhansky and Epling 1944, Guzmán et al. 1993).
The data shown in figure 2 demonstrates that the
3 groups of populations behave in a similar way, yet
the arrangements composing them are different. This
is probably due to the fact that chromosomal arrangements act as super genes, avoiding recombination and
isolating within these structures alleles which could be
better adapted for particular environmental conditions.
On the other hand, comparing populations from
South to North, SC is a very frequent arrangement in
those populations from the South West area like
Zirahuen with 65.7 % (19-20°N), and La Congoja
with 43 % (21-22°N). Its frequency diminishes in the
populations to the East and to the North like in Orizaba,
Veracruz where it was not found and Río Verde, S.L.P.
with a frequency of 14 %. In contrast, PP is a very
frequent arrangement in the North East and decreases
until it disappears in the South.
It is very interesting to see the different patterns
of frequencies of the various arrangements over the
distribution area of the species. They show a distinct
gradient from areas in which they are better adapted,
to those in which they decrease and in some cases
even disappear.
Cline seem to reflect the geophysical patterns of the
region in which they are found (Dobzhansky and Epling
GEOGRAPHICAL GRADIENTES OF POLYMORPHISM
1944, Guzmán et al. 1993, Levine et al. 1995), as well
as the capacity of adaptation of the arrangements and
the time they are present in a population. In this way,
when the three groups of populations were analyzed
they showed that TL is the ubiquitous arrangement
and is the only one found in different frequencies,
but in all populations studied. These results support
the hypothesis that TL is the ancestral arrangement
CONCLUSIONS
In this study, the geographic gradients of the inversion frequencies have revealed that the behaviour of
Mexican populations is parallel to that reported for
populations in the United States, with gradual changes
in neighbouring populations. The main arrangements
present similar behaviour, with high frequencies in some
populations and diminishing smoothly and in some cases
disappearing in farthest populations. TL is the arrangement present in all studied populations supporting the
hypothesis of being the ancestral arrangement.
Nevertheless, it is still necessary to gather more
information about the northern and southern populations in order to be able to have a complete panorama
of the importance of genetic arrangements as components of the different populations, which will lead
to identify the environmental factors determining the
frequencies of the inversions in specific populations
as well as their roles in the evolution of the species.
ACKNOWLEDGMENTS
This paper is dedicated to the memory of Dr.
Alfonso L. de Garay whom I (J. Guzmán) met when
I was finishing my bachelor and was starting to work
on Science. He always encouraged and supported
our group to do our best.
To Biologist Carolina Arceo, Technician Dora Luz
Barrón and Mr. Manuel Quintana for their support
for the collection and processing of the samples. To
CONACyT for the financial support through grant
No. 31736-N.
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25
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