White spotting in the domestic cat (Felis catus) maps near KIT on feline chromosome B1.

Five feline-derived microsatellite markers were genotyped in a large pedigree of cats that segregates for ventral white spotting. Both KIT and EDNRB cause similar white spotting phenotypes in other species. Thus, three of the five microsatellite markers chosen were on feline chromosome B1 in close proximity to KIT; the other two markers were on feline chromosome A1 near EDNRB. Pairwise linkage analysis supported linkage of the white spotting with the three chromosome B1 markers but not with the two chromosome A1 markers. This study indicates that KIT, or another gene within the linked region, is a candidate for white spotting in cats. Platelet-derived growth factor alpha (PDGFRA) is also a strong candidate, assuming that the KIT-PDGFRA linkage group, which is conserved in many mammalian species, is also conserved in the cat.

White spotting pelage and skin phenotypes have been investigated in a variety of mammals. In cats, the white spotting locus (S), appears to affect melanocyte migration. Heterozygous animals (Ss) have a ventral white pattern (Whiting 1919), not spots, as the name would suggest. This ventral white pattern, known as bicolour by cat fanciers, is considered to be dominant to solid colour, with an additive effect of the dominant allele. Full solid colour (ss) is recessive, while the van pattern, in which colour is limited to the head and tail regions, is postulated as homozygous SS (Kuhn & Kroning 1928). Various other white spotting phenotypes show spotting at the ventral midline, such as lockets and belly spots, or white at the extremities, such as gloves and mittens. These other white presentations are anticipated to be allelic to ventral white (Whiting 1919). In addition, dominant white (W), which is associated with blue eye colour and deafness in the cat, may be allelic to white spotting (Whiting 1919; Bergsma & Brown 1971); however, epistasis has complicated allele assignments.

The gene(s) responsible for white spotting in the domestic cat have not yet been identified. In several mammals, mutations in loci including EDNRB, KIT and PDGFRA (Smith ; Stephenson , 1998) have been shown to be causative for the dominant white and/or the white spotting phenotypes. We hypothesized that mutations in EDNRB, KIT, or PDGFRA control at least one type of white spotting pattern in the domestic cat, and we performed a linkage analysis of ventral (bicolour) white spotting using a panel of five microsatellites spanning these three genes.

An extended pedigree of 114 cats segregating for white spotting (Fig. S1) from the WALTHAM Centre for Pet Nutrition (Melton Mowbray, Leics, UK) was used for the linkage analysis. Three white spotting cat phenotypes were assigned according to the white pattern grading system proposed by Robinson (1959): solid, bicoloured and van (Fig. 1). Solid coloured cats had no evidence of white spotting. Bicolour cats had a ventral white pattern with approximately 50% or more of the coat having pigment. Van patterned cats had small spots of colour near the head and tail, and occasionally one or a few pigmented spots on the dorsal or lateral side; hence a more extreme white expression than bicolour. Buccal cells were obtained non-invasively by swabbing the internal cheek of each cat with a cytological brush. DNA was extracted as previously described (Oberbauer ).

Figure 1

White spotting classifications in the WALTHAM pedigree. (a) High white or van patterned, (b) bicolour pattern and (c) solid colour pattern.

Five feline-derived microsatellites (FCA097, FCA149, FCA152, FCA229 and FCA453) that flanked KIT and EDNRB were selected from the feline linkage (Menotti-Raymond , 2003) and radiation hybrid (Murphy ) maps. PDGFRA is not mapped in the cat; however, a KIT-PDGFRA linkage group is found in several mammalian species. Thus, the analysis of markers near KIT would also likely implicate PDGFRA in the cat. Three of the markers (FCA097, FAC149 and FCA152) were on feline chromosome B1 within close proximity to the KIT-PDGFRA region, whereas two of the markers (FCA229 and FCA453) were on feline chromosome A1 near EDNRB. Genotyping of these markers was as performed as described by Grahn . Two-point linkage between the microsatellites and the white spotting locus was conducted as previously described (Lyons ) using the LINKAGE software program (Lathrop ). The expression of bicolour and van was highly variable, so a clear distinction between bicolour vs. van was difficult for a few cats. Thus, the linkage analysis was conducted by coding the non-white, solid coloration as a homozygous recessive trait with complete penetrance and non-variable expression.

All pairwise analyses are presented in Table 1. Significant linkage was found between microsatellite FCA097 and white spotting (Z = 9.072, θ = 0.04). Pairwise analyses supported linkage of FCA149 and FCA152 to FCA097. In addition, FCA149 and FCA152 appeared to be linked to the white spotting locus, although the LOD scores were not significant. FCA097, which we predicted to be 4 cM from white spotting, was previously demonstrated to be approximately 22.15 cM centromeric to KIT (Menotti-Raymond ). PDGFRA has not been localized to feline chromosome B1; however, the KIT-PDGFRA linkage group is highly conserved in other species. Additional markers would help to resolve the genetic map of the cat in this region and to better define the positions of KIT, PDGFRA and white spotting.

Table 1

LOD scores for pairwise comparisons between white spotting and microsatellite markers.

		Recombination rate, Θ
Marker 1	Marker 2	0.00	0.01	0.05	0.10	0.20	0.30	Max LOD, Z	Θ at Max LOD, Z	Θ at Z = −2.0
Spotting	FCA097	−∞	8.52	9.05	8.53	6.75	4.54	9.07	0.04	–
Spotting	FCA149	−∞	0.23	1.85	2.15	1.85	1.22	2.16	0.11	–
Spotting	FCA152	−∞	−5.79	−0.60	1.27	2.16	1.70	2.16	0.20	–
Spotting	FCA229	−∞	−13.13	−6.96	−4.36	−1.98	−0.87	–	–	0.20
Spotting	FCA453	−∞	−17.82	−9.09	−5.62	−2.59	−1.13	–	–	0.23
FCA097	FCA149	−∞	0.24	4.84	5.95	5.62	4.06	6.07	0.13	–
FCA097	FCA152	−∞	−1.73	4.28	5.98	6.12	4.64	6.35	0.15	–
FCA149	FCA152	−∞	−5.58	3.62	6.43	7.23	5.73	7.33	0.17	–
FCA299	FCA097	−∞	−28.55	−13.94	−8.29	−3.60	−1.62	–	–	0.27
FCA299	FCA149	−∞	−30.11	−15.22	−9.23	−3.97	−1.61	–	–	0.28
FCA299	FCA152	−∞	−33.62	−16.91	−10.32	−4.63	−2.01	–	–	0.30
FCA453	FCA097	−∞	−24.99	−11.62	−6.40	−2.12	−0.51	–	–	0.21
FCA453	FCA149	−∞	−33.27	−16.39	−9.65	−3.84	−1.34	–	–	0.27
FCA453	FCA152	−∞	−28.74	−13.39	−7.42	−2.50	−0.61	–	–	0.22

Additionally, two markers on cat chromosome A1 near EDNRB were excluded for linkage with white spotting (Z ≤ −2.0, θ ≤ 0.199 for FCA229; Z ≤ −2.0, θ ≤ 0.233 for FCA453). Based on the radiation hybrid map, one marker, FCA229, is 5.05 cM from EDNRB; thus, the excluded region surrounding this marker suggests that EDNRB does not influence the white spotting phenotype in cats.

This study examined a closed cat colony. A caveat to using a closed colony or family is that the linkage between microsatellites and white spotting could be specific to that colony or family. White spotting is a heterogeneous phenotype, and similar colour patterns found in other families may be caused by different genes. Testing for linkage in families of different breeds could provide additional evidence for KIT or PDGFRA as candidate genes for white spotting in cats. Further investigations of the genes near FCA097 are necessary to provide conclusive evidence of their contribution to white spotting in the domestic cat.