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Final Report Summary - OXIDASPERM (Sexual Selection and the Evolution of Sperm Competitiveness: The Cost and the Avoidance of Oxidative Stress in Sperm Cells)

Females are sexually promiscuous in most mammalian species. Female promiscuity results in the ejaculates from rival males competing to fertilize the same ova, a process referred to as sperm competition. Sperm competition is a powerful selective force, which has shaped many reproductive traits at the physiological, cellular and molecular levels. Given that the degree of female promiscuity differs among species, the levels of sperm competition, and thus the evolved responses associated to sperm competition, also differ among species. An almost universal response to increased levels of sperm competition is to increase testes size and sperm production. The first evolutionary response to increased levels of sperm competition, however, seems to be an increase in sperm swimming speed. Rodent species with higher levels of sperm competition contain more ATP in their sperm, which is associated with an increase in sperm swimming velocity.
The general aim of this project was to elucidate the role of sexual selection in the evolution of traits that enhance sperm competitiveness at the molecular, cellular, morphological and physiological levels. The specific aims of this project were a) to understand the potential negative consequences of adaptive increase in sperm swimming speed in terms of oxidative damage, and b) the counter-measures that evolve to protect the sperm membrane and DNA integrity against such oxidative damage. We tackled these specific aims by addressing the following objectives using as models closely related species of rodents that differ in their levels of sperm competition:

Objective 1. NEGATIVE CONSEQUENCES OF INCREASED SPERM METABOLISM DUE TO INCREASED SWIMMING SPEED
Sperm motility is directly dependent upon the availability of energy obtained by enzymatic decomposition of ATP. Therefore, when facing higher levels of sperm competition, males able to generate more energy (ATP) per individual spermatozoon would likely be favoured by selection. This increase in sperm metabolism may lead to a major production of ROS (objective 1.1), increased peroxidation of membrane lipids (objective 1.2) and higher levels of DNA damage (objective 1.3).

Objective 1.1. Are sperm from species with high sperm competition levels exposed to higher levels of ROS?

Hypothesis: The levels of intracellular and intramitochondrial ROS are higher in species with higher levels of sperm competition
Approach to be carried out: Comparing sperm ROS production under different physiological conditions in species that differ in their levels of sperm competition.

Objective 1.2. Do sperm from species with high sperm competition levels suffer more lipid peroxidation?

Hypothesis: The increase of sperm metabolism in species with high levels of sperm competition leads to a higher incidence of lipid peroxidation. Contrarily, levels of lipid peroxidation may be similar across species regardless of their different levels of sperm competition; this would be the case if species with higher levels of sperm competition have developed counter-measures to prevent oxidative stress (e.g., by reducing the proportion of PUFAs in the membrane; see objective 2).
Approach to be carried out: Measure the level of lipid peroxidation by spectrophotometric determination of thiobarbituric acid reacting substances (TBARS) in relation to different sperm competition levels.

Objective 1.3. Do sperm from species with high sperm competition levels experience more sperm DNA damage?
Hypothesis: Sperm competition may lead to higher levels of sperm DNA damage as a consequence of higher levels of ROS production.
Approach: Quantify the levels of DNA damage using the Sperm Chromatin Structure Assay (SCSA®) in several species that differ in their levels of sperm competition.
Work performed: All collected samples were processed using SCSA. The resulting data were analysed and published. The main finding is that in species with high levels of sperm competition there is more DNA damage.

Objective 2. PROTECTIVE COUNTER-MEASURES TO PREVENT OXIDATIVE STRESS
Males may show adaptations aimed at minimizing ROS-induced oxidative damage to sperm structures. One plausible mechanism is that males may have a greater capacity to defend sperm from ROS through increased antioxidant responses (objective 2.1). The composition of the sperm membrane can also be modified to minimize the deleterious effects of ROS (objective 2.2). Finally, a higher condensation of DNA in the sperm head can decrease the damage suffered by DNA in the presence of ROS (objective 2.3).

Objective 2.1. Do sperm from species with high sperm competition levels have better antioxidant defences?
Hypothesis: The increase in oxidative damage associated with increased levels of sperm competition will be counter-balanced by an increased production of antioxidants.
Approach to be carried out: Quantify antioxidant activity in sperm and fluids from accessory glands.

Objective 2.2. Does the composition of the sperm plasma membrane change in relation to sperm competition levels?
Hypothesis: To protect sperm from increased levels of oxidative damage, species with high levels of sperm competition will have sperm with lower levels of unsaturated fatty acids in membrane lipids. Phospholipid composition and cholesterol content in sperm membranes may also vary.
Approach: Examine possible differences in phospholipid, fatty acid and cholesterol composition in different species in relation to their levels of sperm competition.
Work performed: Samples were processed and analysed. Two publications were produced. The main finding is that an increase in the levels of sperm competition leads to a reduction in the percentage of polyunsaturated fatty-acids (PUFAS) in the sperm membranes. Given that PUFAs are the most sensitive fatty acids to lipid peroxidation, a decrease in the percentage of PUFAs can offer protection against oxidative damage.

Objective 2.3. Does the degree of DNA compaction within the sperm head change in relation to sperm competition levels?

Hypothesis: Species with high levels of sperm competition will have sperm with higher DNA compaction within the sperm head. Such compaction should reduce the access and thus negative effect of ROS on DNA.
Approach to be carried out: Explore whether sperm competition favours the evolution of more compacted sperm DNA by performing cytochemical tests with aniline blue and chromomycin A3 (CMA3), and flow cytometric analysis with CMA3. Chromatin thiol status can be assessed by flow cytometry using monobromobimane (mBBr).

Contact

Ian Pennington, (LEAR)
Tel.: +441519046471
E-mail

Subjects

Life Sciences
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