Narrow Hybrid Zone Between Two Chromosomal Races of the Sceloporus grammicus Complex: Analysis of Ma

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Narrow Hybrid Zone Between Two Chromosomal Races
of the Sceloporus grammicus Complex Analysis of
Male Reproductive Success

• George Dekki
• Department of Biology

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Sceloporus grammicus
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Sceloporus grammicus Complex

• The species is distributed from Mexico to
  southern Texas.
• The lizards live at high elevations ranging from
  2400 to 3200 m (7000-10000 ft).
• The females are viviparous.
• Territorial males with a harem of 5-7 females.
• In 1959, Smith characterized the complex as
  morphologically one single species with no
  observable chromosomal differences.

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Chromosomes of Sceloporus grammicus 2n32
(Standard)
Pr. 1
Pr. 2
Pr. 3
Pr. 5
Pr. 4
Pr. 6
Macrochromosomes
1
2
3
7
4
5
8
6
Y
X2
Microchromosomes
X1
sex chromosomes
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The Cytotypes of S. grammicus

• At least eight different chromosomal races, or
  cytotypes have been described, with diploid
  numbers ranging from 2n 32- 46.
• Cytotypes correspond to incipient species.

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Sceloporus grammicus cytotypes
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Distribution of the Sceloporus
grammicus cytotypes
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What is a Hybrid Zone?

• Hybrid zones occur when two formerly allopatric
  populations have become genetically
  differentiated and expand so that they meet and
  interbreed.
• The result is some offspring of mixed ancestry.
• Hybrid zones provide a wealth of information on
  understanding genetically distinct species and
  subspecies.

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Hybrid Zones

• Seven different hybrid zones were detected in
  central Mexico between different combinations of
  S. grammicus cytotypes.
• A HS (32) X FM3 (38)
• B F5 (34) X FM2 (46)
• C, G LS (32) X FM2 (46)
• D, F F6 (34) X HS (32)
• E LS (32) X F6 (34)

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Tulancingo Hybrid Zone

• This is a secondary contact zone located near
  Tulancingo, Hidalgo, Mexico.
• Extreme chromosomal divergence exists between the
  populations hybridizing.
• The two populations, termed F5 (2n 34) and FM2
  (2n 46) coexist and mate with one another.

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Tulancingo Hybrid Zone
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Tulancingo Hybrid Zone
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The Big Question

• I would like to determine if actual hybrids are
  produced.
• If hybrids are produced, are they able to produce
  viable offspring?
• Also, is there any reproductive cost associated
  with this new generation of offspring?

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The Use of Microsatellites

• Relatively short tandem repeats of DNA (2-6 bp
  long).
• Abundant, highly polymorphic, distributed
  throughout the genome.
• Alleles differ in number of copies of repeats.
• Length of polymorphisms amplified with Polymerase
  Chain Reaction (PCR).
• Ease and accuracy of microsatellites allow us to
  determine paternity's and genotypic variation
  among the different lizard cytotypes.

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Isolated Microsatellites
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Microsatellite Region
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Hybrid Index Score (1 14)


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Habitats of the Hybrid Zone
Typical vegetation for the FM2
populations (drier conditions)
Typical vegetation for the F5
populations (oak trees)
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The F1 Hybrid

• The offspring of the F5 and FM2 lizards is termed
  the F1 hybrid.
• Reed et al. (1995) established that F1 hybrids
  experience lower fitness than either parental or
  backcrossed individuals.
• Presence of recombinant individuals in the hybrid
  zone indicates that F1 hybrids are fertile (Reed
  et al., 1995).

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Lizard Collection

• Collected in June and July of 1989 and again in
  February of 1991.
• Lizards were collected at times when the females
  were gravid (Sites et al., 1991).
• Lizards collected by hand, noose, or by stunning
  with rubber bands.
• Once captured, lizards were assigned a number
  attached by a toe clipping.
• Capture point was marked on the site in one of 28
  quadrants.

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Lizard Collection
Lizards are captured by way of stunning with an
elastic band. The band must hit the lizard at
an angle of its head.
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Gravid Female
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Sample Preparation

• All lizards were captured alive and were
  processed for mitotic and meiotic karyotypes in
  the field.
• Samples were stored in liquid nitrogen for future
  use in the laboratory.
• DNA was extracted from the lizards and
  microsatellite region amplified using PCR.
• The DNA was then run on polyacrylamide gels and
  scored to determine paternity's and genotypic
  differences.

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Reproductive Efficiency
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Sceloporus grammicus Microsatellites
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Conclusions

• At the present time, we are still running gels.
• Hybrids are produced, and they are able to
  produce viable offspring
• We have already determined that the hybrid
  females are not as successful at reproduction
  when compared to the pure females.

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Conclusions

• With the information the gels will give us, we
  will be able to determine whether the hybrid
  males do get to reproduce, and if so are they
  indeed less successful at reproduction?
• What is the reproductive cost for hybrids?

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Acknowledgements

• I would like to thank Dr. Elisabeth
  Arévalo,
• Joel Ferreira, Stephanie Newton, and Melany
  Gomes.