Craniofacial Shape Variation Among Four Laboratory Rat Strains

Forward genetics begins with characterizing the phenotypic traits of interest and moves toward identifying genes that may control the expression of that particular phenotype. Objective: The first step of this line of research, which is the focus of the present study, is to identify inter- and intra- strains variations of the craniofacial complex in different rat strains. Methods: Four rat strains: Sprague-Dawley (SD), Brown-Norway (BN), Holtzman (H), and Wistar-Kyoto (WK) were selected. 4 male (m) and 4 female (f) adult rats were used for each strain. 31 anatomical landmarks were identified in lateral and dorsal head films (21 cranial/nasomaxillary, 10 mandibular). The digitized and landmark fitting programs were written in IDL 5.4 (Interactive Data Language, Research Systems, Inc., CO), and the x and y coordinates of these landmarks were identified. Procrustes algorithm was used to analyze shape differences in mm. Results: The total intra-gender shape variation was: BNm: 11 ± 2, BNf: 14 mm ± 5, SDm: 19 mm ± 6, SDf: 13 mm ± 4, Hm: 15 mm ± 3 Hf: 16 mm ± 6 and WKm: 11 ± 2, WKf: 14 mm ± 4. The total inter-gender shape variation was: H: 18 mm, BN: 15 mm, SD: 18 and WK: 12 mm. The total inter-strain shape variation was: BN vs. WK: 25 mm, H vs. WK: 23 mm, SD vs. WK: 22 mm, BN vs. SD: 22 mm, H vs. BN: 21 mm, H vs. SD: 12 mm. The SD and H showed the most inter-gender shape variation, while WK showed the least. Conversely, SD showed the least inter-strain shape differences while WK showed the greatest. Conclusion: There is considerably greater shape variation inter-strain than intra-strain, except when comparing SD vs. H. When using SD or H for craniofacial research, gender differentiation should be made, while if using WK, no differentiation is necessary.