Example of Herpetological Research in Colorado

BL/ENVS 476: Colorado Flora and FaunaSeptember 19, 2011

A Fundamental Problem• Sexual reproduction is predominant in vertebrates• The common perception is that long-term evolutionary

success is based on phenotypic variability generated by genetic recombination.

• But there are many parthenogenetic lizards that exhibit ecological success; e.g., Aspidoscelis tesselata.

• Alternatives to sexual reproduction in vertebrates:• Hybridogenesis and gynogenesis: in a few fish species

Parthenogenesis: in a few lizard species• The purpose of this research was to test the idea that genetic

recombination is critical for phenotypic variation, using parthenogenetic and sexual species of the lizard genus Aspidoscelis.

A. Aspidoscelis marmorataB. And C. A. gularis septemvittataD. A. tesselata E (2n)F. A. tesselata CE. A. tesselata DG. A. sexlineataH. A. neotesselata (3n)

How do you reproduce without males?• 1. Premeiotic doubling of chromosome number

– DNA synthesis– A. tesselata: 46 single-stranded chromosomes (46 DNA molecules)– Endoreplication 46 double stranded chromosomes– Centromeres separate: 92 single-stranded chromosomes

• 2. Provide each chromosome with a genetically identical synaptic partner.– DNA synthesis– 92 single-stranded chromosomes 92 double-stranded

chromosomes.• 3. Undergo the two meiotic divisions as found in sexual species.

– Synapsis and crossing over with “sister” (identical) chromosomes• 1st division: 92 double-stranded chromosomes /cell 46 double-

stranded chromosomes/cell• 2nd division: 46 double stranded chromosomes/cell 46 single

stranded chromosomes/cell• One becomes an ovum. genetically identical with mother

Patterns of Morphological Variability• 1. Among “uniclonal” and “multiclonal” groups of two color pattern

classes of A. tesselata– Multiclonal = more than one allele for a particular gene locus– Glucose-6-phosphate isomerase (GPI)– Multiclonal: GPI ab and GPI ac– Uniclonal: GPI ac

• 2. Between each of four geographically disjunct groups of A. tesselata and a sympatric sexual species– Controlling for environmental effects on phenotypic variation

• 3. Among the four species– Can the variability of the parthenogenetic species exceed the

variability of a sexually reproducing species?• The phenotypic characters used are quantitative: GAB, FP, COS, LSG, SDL• Meristic characters: counts

The pattern of variability has to be simplifiedDone with multivariate statisticsPrincipal components analysisUses variance/covariance relationships among charactersEstablishes coefficients to multiple times the values of the original charactersNew variables are produced called principal componentsLinear compounds of the original meristic character scores and coefficientsConcentrate as much of the variation of the original variables in a reduced numberof new variables: principal components.Each specimen has score for principal component 1 and principal component 2.Scores are plotted to show patterns of variation.Variances of the principal component scores can be compared for relative

variability.

Do “multiclonal” groups A. tesselata express greater variability than “uniclonal” groups?

Multiclonal: more thanone GPI allele

Uniclonal: one GPI allele

Example of verification

Are differences in phenotypicvariability related to reproductive mode?

Are differences in phenotypicvariability related to reproductive mode?

How does A. tesselata rank on a scale of relative variability with sexual A. sexlineata, A. marmorata, and A. gularis septemvittata?

Conclusions• Aspidoscelis tesselata is organized as a collection of

independent mother-daughter arrays.• Cohesiveness achieved by ecological constraints.• This would mean that the relative phenotypic variability of a

particular parthenogenetic groups should not be predictable by reproductive mode, color pattern class, or geographic location.

• This was confirmed by the present study.• Presumably, phenotypic variation increases as development is

variously modified in clones produced by random mutation.