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Baumann, Hannes; Rosales Casián, Jorge A; Conover, David O (2012): (Appendix) Vertebral number of topsmelt, Atherinops affinis, adults and offspring from different latitudes along the North-American Pacific coast. doi:10.1594/PANGAEA.787095, Supplement to: Baumann, H et al. (2012): Contrasting latitudinal variations in vertebral number and sex determination in Pacific versus Atlantic Silverside fishes. Copeia, 2012(2), 341-350, doi:10.1643/CG-10-189

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Organisms that are distributed across spatial climate gradients often exhibit adaptive local variations in morphological and physiological traits, but to what extent such gradients shape evolutionary responses is still unclear. Given the strong natural contrast in latitudinal temperature gradients between the North-American Pacific and Atlantic coast, we asked how increases in vertebral number (VN, known as Jordan's Rule) with latitude would differ between Pacific (Atherinops affinis) and Atlantic Silversides (Menidia menidia), two ecologically equivalent and taxonomically similar fishes with similar latitudinal distributions. VN was determined from radiographs of wild-caught adults (genetic + environmental differences) and its genetic basis confirmed by rearing offspring in common garden experiments. Compared to published data on VN variation in M. menidia (a mean increase of 7.0 vertebrae from 32 to 46°N, VN slope = 0.42/lat), the latitudinal VN increase in Pacific Silversides was approximately half as strong (a mean increase of 3.3 vertebrae from 28 to 43°N, VN slope = 0.23/lat). This mimicked the strong Atlantic (1.11°C/lat) versus weak Pacific latitudinal gradient (0.40°C/lat) in median annual sea surface temperature (SST). Importantly, the relationship of VN to SST was not significantly different between the two species (average slope = -0.39 vertebrae/°C), thus suggesting a common thermal dependency of VN in silverside fishes. Our findings provide novel support for the hypothesis that temperature gradients are the ultimate cause of Jordan's Rule, even though its exact adaptive significance remains speculative. A second investigated trait, the mode of sex determination in Atlantic versus Pacific Silversides, revealed patterns that were inconsistent with our expectation: M. menidia displays temperature-dependent sex determination (TSD) at low latitudes, where growing seasons are long or unconstrained, but also a gradual shift to genetic sex determination (GSD) with increasing latitude due to more and more curtailed growing seasons. Sex ratios in A. affinis, on the other hand, were independent of latitude and rearing temperature (indicating GSD), even though growing seasons are thermally unconstrained across most of the geographical distribution of A. affinis. This suggests that additional factors (e.g., longevity) play an important role in shaping the mode of sex determination in silverside fishes.
Median Latitude: 35.255000 * Median Longitude: -119.287500 * South-bound Latitude: 28.250000 * West-bound Longitude: -124.200000 * North-bound Latitude: 43.380000 * East-bound Longitude: -114.080000
Date/Time Start: 2008-05-01T00:00:00 * Date/Time End: 2009-05-01T00:00:00
Coos_Bay * Latitude: 43.380000 * Longitude: -124.200000 * Date/Time: 2008-06-19T00:00:00 * Location: North Bend, Oregon
Elkhorn_Sl * Latitude: 36.820000 * Longitude: -121.740000 * Date/Time Start: 2008-05-16T00:00:00 * Date/Time End: 2009-05-09T00:00:00 * Location: South Marsh, California
Lag_Manuela * Latitude: 28.250000 * Longitude: -114.080000 * Date/Time Start: 2009-04-22T00:00:00 * Date/Time End: 2009-04-24T00:00:00 * Location: Baja California coast, Mexico
#NameShort NameUnitPrincipal InvestigatorMethodComment
1Event labelEvent
2PopulationPopul#Baumann, HannesPopulation number ranked by latitude from south to north
3OriginOriginBaumann, Hannes
4Longitude of eventLongitude
5Latitude of eventLatitude
7Sample typeSamp typeBaumann, Hannes
8SexSexBaumann, Hannes
9Temperature, technicalT tech°CBaumann, HannesT at which offspring was reared
10IndividualsInd#Baumann, Hannesrunning no. of individuals
11Atherinops affinis, total lengthA. affinis TLmmBaumann, Hannes
12Atherinops affinis, vertebral numberA. affinis vertebae#Baumann, Hannes
5787 data points

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