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Appendix S1. Coherence between pairing success measured in the laboratory and observed pairing
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frequency in the field
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Under laboratory conditions, amphipods originating from MOTUs genetically diverging by
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approximately 3% formed an amplexus in 85% of trials. This means that, when amphipods from these
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MOTUs meet, they have a 0.85 probability of pairing up. Their probability of meeting up depends on
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their respective frequency in the population they originate from. Consider p(A) as being the
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frequency of the MOTU A in the population and p(B) as the frequency of the MOTU B. Note that
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these frequencies do not need to sum up to one, as other MOTUs may also be present in the
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population. The probability for an A individual of meeting a B individual is Pm = 2 × p(A) × p(B). The
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sex ratio is assumed to be even (which is the case in the populations we sampled) so Pm does not
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depend on whether a male A encounters a female B or the opposite. Probability of pairing between
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individuals A and B is thus Pm = 2 × p(A) × p(B) under random pairing and Pnon-random = 2 × p(A) × p(B) ×
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0.85 when individuals follow the same rules for pairing as in our laboratory experiment. Determining
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whether individuals do or do not discriminate between potential partners involves comparing the
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frequencies of observed MOTU A/MOTU B pairs to the odd of finding such pairs if individuals were
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pairing without discrimination. This is done by computing an odds ratio as a measure of effect size of
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the difference between observed and expected frequencies for the considered pairing combination.
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Considering that we sample MOTU A/MOTU B pairs from a natural population where individuals do
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follow the rules described in our experiment, how many pairs would we need to sample to detect
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that individuals from MOTU A and MOTU B only mate 85% of the time? Figure S2 represents the
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odds ratio of the difference between observed frequency of pairing and the frequency of pairing
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expected under no discrimination between A and B individuals, as a function of the number of pairs
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sampled. For this example, p(A) = p(B) = 0.5. Expected frequency of MOTU A/MOTU B pairs is thus Pm
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= 2 × 0.5² = 0.5 while the observed frequency should be Pnon-random = 2 × 0.5² × 0.85 = 0.425. In order
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to be confident about the statistical significance of the difference between observed and expected
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pairing frequencies (i.e. when the confidence interval of the odd ratio does not cross 1), a minimum
1
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of 400 amphipod pairs should be sampled (Fig. S2). Biasing the frequency of one MOTU at the
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expense of the other MOTU or decreasing the frequencies of both MOTUs only increases the number
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of pairs needed to detect a statistically significant effect (result not shown). In the populations we
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studied, it is possible that amphipods from MOTU genetically diverging by approximately 3% paired
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up with a 0.85 probability. However, as we sampled about 100 couples per population, it is unlikely
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that we could have detected a statistically significant difference between observed and expected
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pairing frequencies between these MOTUs. This may explain the discrepancy between our field
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observations and laboratory results.
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Fig. S2. Odds ratio of the difference between observed frequency of pairing and the frequency of
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pairing expected when individuals originating from genetically divergent MOTUs (~ 3%) pair up
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without discrimination, as a function of the number of sampled pairs. Red lines represent the 95%
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confidence interval for odd ratios. p(A) = p(B) = 0.5
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2