Molecular mechanisms of heavy metal tolerance and evolution in invertebrates

Abstract

Following the genomics revolution, our knowledge of the molecular mechanisms underlying defenses against stress has been greatly expanded. Under strong selective pressure many animals may evolve an enhanced stress tolerance. This can be achieved by altering the structure of proteins (through mutations in the coding regions of genes) or by altering the amount of protein (through changes in transcriptional regulation). The latter type of evolution can be achieved by substitutions in the promoter of the gene of interest (cisregulatory change) or by altering the structure or amount of transcriptional regulator proteins (trans-regulatory change). The metallothionein system is one of the best studied stress response systems in the context of heavy metals. Metallothionein expression is assumed to be regulated by metal transcription factor 1 (MTF-1); however, up to now the involvement of MTF-1 has only been proven for some vertebrates and Drosophila. Data on invertebrates such as nematodes and earthworms suggest that other mechanisms of metallothionein induction may be present. A detailed study of Cd tolerance was done for a species of soilliving springtail, Orchesella cincta. The metallothionein gene of this species is overexpressed in metal-exposed field populations. Analysis of the metallothionein promoter has demonstrated extensive polymorphisms that have a functional significance, as shown in bioreporter assays. In a study comparing 20 different populations, the frequency of a high-expresser promoter allele was positively correlated with the concentration of metals in soil, especially Cd. The springtail study shows that cis-regulatory change of genes involved in the cellular stress response may contribute to evolution of metal tolerance.

Effect of lead and cadmium ions upon the pupariation and morphological changes in Calliphora vicina (Diptera, Calliphoridae)

“…in volcanic rocks, ores, various deposits created by industrial activity (Sharma and Agrawal, 2005;Florea and Busselberg, 2006). Both biologically necessary and undesirable metals can be toxic for living organisms if they are present in doses which exceed the critical level (Janssens et al, 2009).…”

The amphipod Orchomenella pinguis – A potential bioindicator for contamination in the Arctic

“…But, part of the explanation can also be that different population traits and strategies have been selected at these two sites, resulting in different population characteristics, hereunder skewed sex ratios. Selection toward tolerant populations has previously been demonstrated in laboratory exposure systems (e.g., Luoma et al, 1983;Klerks and Levinton, 1989;Galletly et al, 2007;Janssens et al, 2009), where for example Klerks and Levinton (1989) found population-level resistance acquired after only one to four generations in an oligochaete cultured in the presence of cadmium. Anthropogenically impacted sediments are however rarely contaminated with single chemicals, and resistance to contaminant mixtures appears to be more difficult to achieve than resistance to single substances Klerks (1999).…”

Variability of Heavy Metals in Wild Catfish Clarias gariepinus Collected South and North Cairo Area. II. Essential Heavy Metals.

“…Iron, copper, zinc and manganese are classified as essential heavy metals in human diet because they all play significant roles in metabolic processes (Janssens et al, 2009). These metals are also known to help in the growth and production of aquatic life.…”