Low reproductive isolation and highly variable levels of gene flow reveal limited progress towards speciation between European river and brook lampreys.

Ecologically based divergent selection is a factor that could drive reproductive isolation even in the presence of gene flow. Population pairs arrayed along a continuum of divergence provide a good opportunity to address this issue. Here, we used a combination of mating trials, experimental crosses and population genetic analyses to investigate the evolution of reproductive isolation between two closely related species of lampreys with distinct life histories. We used microsatellite markers to genotype over 1000 individuals of the migratory parasitic river lamprey (Lampetra fluviatilis) and freshwater-resident nonparasitic brook lamprey (Lampetra planeri) distributed in 10 sympatric and parapatric population pairs in France. Mating trials, parentage analyses and artificial fertilizations demonstrated a low level of reproductive isolation between species even though size-assortative mating may contribute to isolation. Most parapatric population pairs were strongly differentiated due to the joint effects of geographic distance and barriers to migration. In contrast, we found variable levels of gene flow between sympatric populations ranging from panmixia to moderate differentiation, which indicates a gradient of divergence with some population pairs that may correspond to alternative morphs or ecotypes of a single species and others that remain partially isolated. Ecologically based divergent selection may explain these variable levels of divergence among sympatric population pairs, but incomplete genome swamping following secondary contact could have also played a role. Overall, this study illustrates how highly differentiated phenotypes can be maintained despite high levels of gene flow that limit the progress towards speciation.

[Results of a new controlled survey of the spine of tractor operators]. / Résultats d'une nouvelle enquête contrôlée sur le rachis des tractoristes.

In order to appreciate the consequence of tractor driving on the incidence of low back pain 72 tractorists (mean age: 35,1 +/- 6,3; mean first year driving (18,6 +/- 4,8), mean duration of driving (15,2 +/- 4,7) were compared to 62 non tractor driving farm workers (mean age: 36,5 +/- 6,5). Subjects were studied during a systematic yearly examination at the regional office for medical prevention. They were not informed of the purpose of the study. All subjects were submitted to a 45 items questionnaire made by one rheumatologist and to a standard physical examination of the dorsal and lumbar spine by another rheumatologist. No X-rays examination were performed during this enquiry. Results were as follows: non specific lowback pain was a frequent complaint in both groups, but was found significantly more frequent in non tractorists (p less than 0,01). Discrimination of non specific low back pain by duration of pain episodes (greater than or equal to 3 months) gave of lower frequency of the symptom which was found similar in both groups. Non tractorists complained more frequently of lumbago and sciatica (p less than 0,05). Identical abnormal physical examination were found in tractorists and non tractorists. Professional consequence were infrequent and not different between tractorists and non tractorists. No correlation were found between results of questionnaire, physical examination and mean duration of tractor driving.