Institute of Botany
Sect. Plant Ecology
Molecular selection and conservation issues of hermaphroditic organisms
A correlation of “neutral” molecular patterns and local habitat characteristics is rarely described in terrestrial organisms. We found a very good example for this process in a selfing hermaphrodite. Cochlicopa lubrica, a hermaphroditic land snail of Europe, shows conspicuous patterns at the enzyme locus aspartate aminotransferase (AAT-1). Homozygous individuals with allelic combination 'a/a' appear frequently in open, dry habitats, whilst homozygous individuals with allelic combination 'b/b' are predominately found in shady, moist habitats. This spatial distribution of homozygous individuals was found within few hundred metres of distance among the two habitat types. Natural selection seems the main force shaping this adaptation. Heterozygous individuals 'a/b' were only rarely detected because continuous self-fertilization drives all progenies towards homozygosity. Interestingly, the two habitat-specific strains 'a/a' and 'b/b' cannot be discriminated by shell or anatomical characters (ref. 1). The results show that selection can directly act upon the spatial distribution of „neutral“ genomic markers (e.g., by linkage disequilibrium of chromosomal regions under high self-fertilization). A similar process was documented in the selfing grass Avena barbata among micro-scaled habitat patches (Hamrick & Holden 1979; Evolution 33:521-533).
Why is a correlation of „neutral“ molecular markers and micro-scaled habitat characteristics so rarely found in terrestrial organisms? This is obscure since marine invertebrates of the litoral provide even better examples for this process (see reviewing table in ref. 1). I speculate that millions of years of habitat stability in the litoral could be one reason why marine genotypes are „better adapted“ to micro-scale regimes than organisms living in terrestrial systems. Terrestrial habitats might be more unpredictable in adaptation processes, and could have a stronger habitat-turnover than the litoral system.
The mating system (self-fertilization versus cross-fertilization) has impact on conservation genetic issues. Generally, the genetic variability is reduced if populations go through severe bottlenecks. Self-fertilizing diploid organisms have a much higher risk of losing allelic variation under bottleneck conditions than cross-fertilizers have. This is even the case, if the frequency of alleles before the bottleneck is identical for both breeding systems (ref. 2). Hence, the genetic potential of self-fertilizing populations needs particular attention in conservation management.
In a further study, we focused on the population profile of the rock living hermaphroditic snail Chondrina avenacea in the Swiss Jura mountains. We found evidence that populations living on small rocks have statistically higher genetic variation than populations living on large and continuous cliffs. We explain this pattern with spatial drift, self-fertilization and restricted gene flow among the populations Thus, even small rock habitats can harbour populations with relatively high genetic variation. This has relevance in the protection of animal and plant populations living on isolated cliff habitats (ref. 3).
(2) Armbruster GFJ & Pfenninger M (2003) -- Simulated bottlenecks and loss of rare alleles: implications on the conservation genetics of two gastropod species -- Journal for Nature Conservation 11: 77-81
(3) Armbruster GFJ, Hofer M & Baur B (2007) -- Effect of cliff connectivity on the genetic population structure of a rock-dwelling land snail species with frequent self-fertilization -- Biochemical Systematics and Ecology (in press)
Why is a correlation of „neutral“ molecular markers and micro-scaled habitat characteristics so rarely found in terrestrial organisms? This is obscure since marine invertebrates of the litoral provide even better examples for this process (see reviewing table in ref. 1). I speculate that millions of years of habitat stability in the litoral could be one reason why marine genotypes are „better adapted“ to micro-scale regimes than organisms living in terrestrial systems. Terrestrial habitats might be more unpredictable in adaptation processes, and could have a stronger habitat-turnover than the litoral system.
The mating system (self-fertilization versus cross-fertilization) has impact on conservation genetic issues. Generally, the genetic variability is reduced if populations go through severe bottlenecks. Self-fertilizing diploid organisms have a much higher risk of losing allelic variation under bottleneck conditions than cross-fertilizers have. This is even the case, if the frequency of alleles before the bottleneck is identical for both breeding systems (ref. 2). Hence, the genetic potential of self-fertilizing populations needs particular attention in conservation management.
In a further study, we focused on the population profile of the rock living hermaphroditic snail Chondrina avenacea in the Swiss Jura mountains. We found evidence that populations living on small rocks have statistically higher genetic variation than populations living on large and continuous cliffs. We explain this pattern with spatial drift, self-fertilization and restricted gene flow among the populations Thus, even small rock habitats can harbour populations with relatively high genetic variation. This has relevance in the protection of animal and plant populations living on isolated cliff habitats (ref. 3).
References
(1) Armbruster G (2001) -- Selection and habitat-specific allozyme variation in the self-fertilizing land snail Cochlicopa lubrica (O.F. Müller) -- Journal of Natural History 35: 185-199(2) Armbruster GFJ & Pfenninger M (2003) -- Simulated bottlenecks and loss of rare alleles: implications on the conservation genetics of two gastropod species -- Journal for Nature Conservation 11: 77-81
(3) Armbruster GFJ, Hofer M & Baur B (2007) -- Effect of cliff connectivity on the genetic population structure of a rock-dwelling land snail species with frequent self-fertilization -- Biochemical Systematics and Ecology (in press)