Chironomus bifurcatus, Wuelker, Wolfgang, Martin, Jon, Kiknadze, Iya I., Sublette, James E. & Michiels, Susanne, 2009

Wuelker, Wolfgang, Martin, Jon, Kiknadze, Iya I., Sublette, James E. & Michiels, Susanne, 2009, Chironomus blaylocki sp. n. and C. bifurcatus sp. n., North American species near the base of the decorus-group (Diptera: Chironomidae), Zootaxa 2023, pp. 28-46 : 36-40

C. bifurcatus sp. n.

Chironomus species a Martin 1979

Chironomus decorus-group species 1 Butler et al. 1995 Chironomus species a Martin 2008

Type material. Holotype, CANADA: Lake Deschêne, near Ottawa, Ontario, egg mass #1 laid 21.IX.66, slide C0.39.2 18M JM, in ZSM.

Other material examined. Canada. Lake Deschéne, Ottawa, Carleton Co., Ontario, from egg masses, 22–23.X.1966 (JM), 5 Sgc with larval heads; 10 imagines 1 (I with Pex, not assoc) 3 I male, 6 I female, 5LP female. Copanspin farm, Dunrobin, Carleton Co., Ontario, 2.V and 24.X.1966 (JM) 7 Sgc with larval heads, 2 LPI male, 1 LI female, 1 LI male, 1 P female. Edge of creek 1/ 2 mile e. Dunrobin, Carleton Co., Ontario, 25.IV.1966 and 25.IV.1967 (JM) 5 Sgc with larval head, 1 PI female, 1 LI male, 1 LPI female, 1 PI male. Rideau River, Carleton Co., Ontario, V–VII.1966 (JM) 2 I male, 2 LPI male, 1 LP male 1 PI male. Bear Creek, nr. Carlsbad Springs, Carleton Co., Ontario, 19.IV.1966 (JM) 3 Sgc with larval head. Barrel drain near Squirrel Rapids, Algonquin Park, Nipissing Co., Ontario, 29.VI.1966 (JM) 1 LPI male. Mile 14.3, Highway 60, Algonquin Park, Nipissing Co., Ontario, 1.VI.1966 (JM) 3 Sgc with larval head, 1 LP male. Brewery Creek, Hull, Quebec, 24.VIII.1970 (JM & J. Mackie) 3 I male, 1P male, 3 I female, 1 female Pex, 1 P female, 9 Pex, 44 Sgc with larval head. U.S.A. Lake Michigan, Epoufette, Michigan, from egg mass #1 laid 21.ix.1966, em. 17–26.x.1966, (J.M.) 4 Sgc with larval head, 2 PI male, 1 P male, 1 I female. Anderson Lake, Clearwater Co., Minnesota, 28.III.1993 ( MGB) 35 Sgc. Turtle Lake, Becker Co., Minnesota, I.1974, III.1974 and 19.IX.1974 ( MGB) 39 Sgc.

Adult specimens are in the collections of CNC and UMN.

Etymology. From the forked appearance of arm G in many polytene chromosome squashes.

Diagnostic characters. This species is presently most accurately identified by the unique banding pattern of the polytene chromosomes. There are unique patterns in all of the chromosome arms, except for arm E. In arm C, one pattern is shared with C. blaylocki , but a second unique pattern also exists. It is not possible to specify a particular diagnostic sequence because specimens may have different combinations of the polymorphic sequences, but their should be one of the species specific sequences of Arms A (bifA1, 2, 3 or 4), B (bifB1 or 2), D (bifD1 or 2), F (bifF1 or 2) and the characteristic arm G.

In Townes (1945), the male of this species keys to “ Tendipes ” (= Chironomus ) decorus ( Johannsen 1905) . It can be differentiated from C. decorus by that species having paler saddle-shaped fascia on terga II–V or VI ( Johannsen, 1905), and with the superior volsella longer and paler and usually slightly widened near the middle. Townes’ Fig. 136A agrees most closely with Johannsen’s type material, while 136B is apparently Chironomus maturus Johannsen, 1918 and 136C is probably this new species; his Fig. 136D is possibly Chironomus whitseli Sublette & Sublette 1974 . The heavier gonostylus shown in his Fig. 136C agrees with the males here associated with this new species. The single male tentatively associated with C. blaylocki is extremely similar but has a darker abdomen and the superior volsella is longer, more evenly tapered from the base and not as angled on the lateral margin ( Fig. 3 c); the phallopodemes of the two species, C. bifurcatus and C. blaylocki , are distinct ( Fig. 3 c, d). The abdominal colour pattern of C. bifurcatus ( Fig. 3 b) is very similar to that of C. blaylocki ( Fig. 3 a) and C. maturus ( Sublette & Sublette 1974, Fig. 2 ); however, the heavier gonostylus and the less strongly hooked superior volsella are distinctive for this species.

Material is not available to definitively separate this species in the pupal stage from other member of the maturus -group. It is readily separated from C. decorus by that species having secondary tubercles on the frontal apotome ( Fig. 6 a, b).

Karyotype ( Fig. 4 )

Chromosome arm combination AB, CD, EF, G (thummi-cytocomplex). Terminal nucleolus in arm G, no nucleolus in long arms. There are normally three BRs, located in similar to positions to those of C. blaylocki . Inversion polymorphism known in chromosome arms A, B and F.

Arm A

There are four sequences of arm A in our samples. A2 and A3 differ from bifA1, by simple inversions, while A4 can be derived from A3 by a further simple inversion (see below).

There is a problem in the location of bands 4ab. The incomplete appearance of group 4 in the Palaearctic decorus-group species C. obtusidens was regarded by Keyl (1961, 1962) as due to structural modification. In contrast, we see bands 4ab transported to another site on the arm. However, this may be due to an independent origin of the arm A banding patterns of C. obtusidens and C. bifurcatus , as outlined below. It is very difficult to derive the C. bifurcatus patterns from the basic arm A pattern, e.g. in C. holomelas Keyl, 1961 , but the bifA2 pattern can be obtained via 11 simple inversions (see below). This scheme indicates that bifA2 is derived by a separate set of inversion steps to those leading to C. obtusidens and C. blaylocki .

Some of the intermediate steps may eventually be found in the arm A patterns of other uncharacterised species of the decorus-group, e.g. C. decorus R&F ( Rothfels & Fairlie 1957) or C. decorus (Martin et al. 1979) .

Arm B

B1 with BR (group 7) in the middle of the arm, which in B2 is transported to a more distal position. The bands distal to the BR in B1 are not group 8, as in blaB1 of C. blaylocki , but rather the bands proximal to the BR of blaB1. Group 8 may be more distally located in the C. bifurcatus patterns.

Arm C

Two patterns are present in arm C. The common C1 has the same sequence, and consequently the same relationship to the common pattern, as in blaC1. The second sequence, C2, is known only as the heterozygote, and differs from C1 by the simple inversion 4i–15

The pattern of bifD1 can be derived from the common pattern of arm D by five inversion steps. It can also be derived from the pattern of C. piger , but this requires six inversion steps. Neither mode of derivation shows any common intermediate sequence with the derivation of blaD1.

Arm F The simplest explanation for the banding pattern in arm F is that it is derived from the F1 pattern of C. blaylocki via a hypothetical intermediate with the simple inversion 6–17. A further three break inversion of this intermediate gives the pattern of bifF1. A second sequence, bifF2, is known only from a single heterozygote and has approximate limits 14–19.

Arm G

Long and stretched, generally partly unpaired like a fork (species name), but may be completely unpaired, possibly due to inversion heterozygosity. There is a virtually terminal nucleolus at the unpaired end, with a nearby BR as in C. blaylocki , and another BR near middle of arm. There is considerable variability in the degree of puffing of the BRs with developmental stage

Inversion polymorphism: In our material polymorphism occurs in arms A, B, C and D, but there is a low frequency of polymorphism for heterochromatin near the nucleolus ( Fig. 5 b) and also medially ( Fig. 4 ) in arm G. As well, there can be supernumerary chromosomes in mid-Western populations ( Fig. 5 b). Arm A polymorphisms are widespread, with both A1 and A3 common in different populations, while A2 is less frequent. In the largest sample, South March, Ontario, the frequencies are A1 = 46.4%, A2 = 13.1%, A3 = 40.4%. Both sequences of arm B are common in Ontario populations (67.6% B1, 32.4% B2 at South March), but B2 is rare in the mid-Western samples. C2 and D1 occur at low frequency in both Ontario and the mid- West (both 1.2% at South March).

Figures

FIGURE 2. Larval head characters of C. blaylocki sp. n. (a – c) and C. bifurcatus sp. n. (d). a, d—mentum and ventromentum, b—antenna, c—mandible.

FIGURE 3. Adult characters of C. blaylocki sp. n. (a, c) and C. bifurcatus sp. n. (b, d, e). a, b—tergal patterns; c, d—genitalia (left) and superior volsella (right); e—variation in superior volsella.

FIGURE 4. Polytene chromosomes of Chironomus bifurcatus sp. n. * — heterozygous heterochromatic band in arm G. Limits of inversion A 4 are not able to be shown. Other symbols as Fig. 1.

FIGURE 5. a—Arm B of C. blaylocki sp. n. heterozygous for B 1.2. The relative position of band groups 7 and 8 in the two homologs is marked. b—Arm G of a mid-Western larvae of C. bifurcatus sp. n. showing strong heterochromatin near the nucleolus, and also the presence of B-chromosomes (Bch). Other symbols as in Fig. 1.

FIGURE 6. Pupal characters of C. bifurcatus sp. n. (a, c, d) and C. decorus Johannsen (b). Frontal apotome of C. bifurcatus (a) compared to that of C. decorus (b) with its secondary tubercles. Caudolateral spurs with different numbers of apical spines (c, d).