Anatomical retraction of the semitendinosus muscle following harvest of the distal semitendinosus tendon for ACL reconstruction.

PURPOSE: Retraction of semitendinosus muscle has been reported after reconstruction of the anterior cruciate ligament with semitendinosus/gracilis-graft. However, very little data exist on the natural variation in side-to-side length symmetry. The purpose of this study was to investigate the side-to-side asymmetry of semitendinosus muscle length in individuals with ACL reconstruction (ACLR) using the semitendinosus/gracilis-graft compared to a group of healthy control subjects to establish the level of retraction that can confidently be ascribed the surgery. METHODS: Eleven subjects aged 30 (19-39) years, with previous unilateral ACLR with the combined semitendinosus/gracilis tendon graft were recruited. Average follow-up was 6.8 years (0.3-13.0) after reconstruction. Ten healthy subjects aged 30 years (23-36) with no previous knee surgery served as controls. Bilateral magnetic resonance imaging (MRI) scans were obtained of the thigh from 60 mm below the knee joint and 700 mm proximal to this point with a slice thickness of 5 mm with 5 mm inter-slice distance. Semitendinosus length was measured on both legs between the distal and proximal musculotendinous junction of the semitendinosus. Length difference between legs was calculated for all participants. Percentage of shortening was expressed relative to the healthy leg. RESULTS: Subjects who had undergone ACLR had on average 81 mm (25%) shortening of the semitendinosus on the reconstructed leg compared to the non-reconstructed side. The healthy subjects all had less than 10 mm difference between legs (< 3%). The side-to-side difference was significantly different between the reconstructed patients and the healthy subjects (p < 0.001). CONCLUSION: This study indicates that retraction larger than 10 mm is a consequence of the tendon harvest and not natural variation. It also supports that persistent retraction of the semitendinosus muscle occurs following harvest of the semitendinosus tendon for ACL graft. LEVEL OF EVIDENCE: Level IV.

Assessing pre- and post-zygotic barriers between North Atlantic eels (Anguilla anguilla and A. rostrata).

Elucidating barriers to gene flow is important for understanding the dynamics of speciation. Here we investigate pre- and post-zygotic mechanisms acting between the two hybridizing species of Atlantic eels: Anguilla anguilla and A. rostrata. Temporally varying hybridization was examined by analyzing 85 species-diagnostic single-nucleotide polymorphisms (SNPs; FST ⩾0.95) in eel larvae sampled in the spawning region in the Sargasso Sea in 2007 (N=92) and 2014 (N=460). We further investigated whether genotypes at these SNPs were nonrandomly distributed in post-F1 hybrids, indicating selection. Finally, we sequenced the mitochondrial ATP6 and nuclear ATP5c1 genes in 19 hybrids, identified using SNP and restriction site associated DNA (RAD) sequencing data, to test a previously proposed hypothesis of cytonuclear incompatibility leading to adenosine triphosphate (ATP) synthase dysfunction and selection against hybrids. No F1 hybrids but only later backcrosses were observed in the Sargasso Sea in 2007 and 2014. This suggests that interbreeding between the two species only occurs in some years, possibly controlled by environmental conditions at the spawning grounds, or that interbreeding has diminished through time as a result of a declining number of spawners. Moreover, potential selection was found at the nuclear and the cytonuclear levels. Nonetheless, one glass eel individual showed a mismatch, involving an American ATP6 haplotype and European ATP5c1 alleles. This contradicted the presence of cytonuclear incompatibility but may be explained by that (1) cytonuclear incompatibility is incomplete, (2) selection acts at a later life stage or (3) other genes are important for protein function. In total, the study demonstrates the utility of genomic data when examining pre- and post-zyotic barriers in natural hybrids.

Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms.

The two North Atlantic eel species, the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata), spawn in partial sympatry in the Sargasso Sea, providing ample opportunity to interbreed. In this study, we used a RAD (Restriction site Associated DNA) sequencing approach to identify species-specific diagnostic single-nucleotide polymorphisms (SNPs) and design a low-density array that combined with screening of a diagnostic mitochondrial DNA marker. Eels from Iceland (N=159) and from the neighboring Faroe Islands (N=29) were genotyped, along with 94 larvae (49 European and 45 American eel) collected in the Sargasso Sea. Our SNP survey showed that the majority of Icelandic eels are pure European eels but there is also an important contribution of individuals of admixed ancestry (10.7%). Although most of the hybrids were identified as F1 hybrids from European eel female × American eel male crosses, backcrosses were also detected, including a first-generation backcross (F1 hybrid × pure European eel) and three individuals identified as second-generation backcrosses originating from American eel × F1 hybrid backcrosses interbreeding with pure European eels. In comparison, no hybrids were observed in the Faroe Islands, the closest bodies of land to Iceland. It is possible that hybrids show an intermediate migratory behaviour between the two parental species that ultimately brings hybrid larvae to the shores of Iceland, situated roughly halfway between the Sargasso Sea and Europe. Only two hybrids were observed among Sargasso Sea larvae, both backcrosses, but no F1 hybrids, that points to temporal variation in the occurrence of hybridization.