Table 3. Intergenic spacer groups and supergroups.

NTS group	Species	Clade	N	Length	Mean P-distance
Supergroup I			72	286–329	0.135±0.010
α	Ensis directus	A	41	321–329	0.011±0.003
β	Ensis macha	A	18	314–318	0.019±0.004
ζ	Ensis magnus, E. siliqua, E. ensis, E. goreensis	E	13	286–315	0.042±0.006
Supergroup II			28	407–965	0.240±0.012
γ	Ensis directus	A	11	444–654	0.023±0.004
δ	Ensis directus	A	4	407	0.002±0.002
η*	Ensis magnus, E. siliqua, E. ensis, E. minor (Chenu)	E	9	893–965	0.046±0.004
θ*	Ensis directus, E. macha, E. minor (Dall)	A	4	926–960	0.127±0.008
Supergroup III			14	405–620	0.141±0.009
ɛ 1	Ensis macha	A	6	603	0.011±0.003
ɛ 2	Ensis macha	A	5	618–620	0.004±0.002
ξ*	Ensiculus cultellus		3	405	0.010±0.004
Supergroup IV			8	355–550	0.241±0.013
μ	Pharus legumen		3	548–550	0.005±0.002
o*	Siliqua patula		2	355	0
λ*	Pharus legumen		3	419–420	0
Supergroup V
ι*	Ensis macha	A	1	776
Supergroup VI			2	209–369	0.077±0.018
π*	Siliqua patula		1	369
ρ*	Siliqua patula		1	209
Supergroup VII			2	283–332	0.366±0.029
κ	Pharus legumen		1	332
ν*	Ensiculus cultellus		1	283
IGS group	Species		n	Length	Mean P-distance
Supergroup Ensis–Ensiculus			15	222–422	0.203±0.015
Ensis spp.	Ensis directus, E. macha, E. minor (Dall, 1899)		13	225–231	0.177±0.014
	Ensis magnus, E. siliqua, E. ensis, E. minor (Chenu, 1843)
Ensiculus cultellus	Ensiculus cultellus		2	421–422	0.002±0.002
Supergroup Pharus–Siliqua			5	236–342	0.193±0.017
Siliqua patula	Siliqua patula		2	236	0.064±0.015
Pharus legumen	Pharus legumen		3	342	0.007±0.004

Abbreviations: A, American clade; E, European clade (Ensis phylogenetic clades according to Vierna et al. (unpublished data)); IGS, intergenic spacer (downstream a U1 small nuclear RNA coding region); n, sample size, NTS, nontranscribed spacer (intergenic spacer downstream a 5S ribosomal RNA coding region). Asterisks (^*) indicate nontranscribed spacers linked to U1 small nuclear DNA;