Thalassomya paraskevae Makarchenko, Semenchenko, Palatov & Lisanovskaya, 2024

Michailova, Paraskeva, 2025, External morphology of the larva and polytene chromosomes of Thalassomya paraskevae Makarchenko, Semenchenko, Palatov & Lisanovskaya, 2024 (Diptera, Chironomidae: Telmatogetoninae) from the Bulgarian Black Sea Coast, Zootaxa 5613 (3), pp. 555-566 : 556-564

Larva of Thalassomya paraskevae Makarchenko, Semenchenko, Palatov & Lisanovskaya

The length of the larva is around 9–10 mm in length and the general coloration is green. The head is dark brown to black, heavily sclerotized and separated from the body by a dark to black border. A small eye spot is present on each side of the head, behind the mandibles. The mandible and mentum are black. The mandible ( Figs. 2 A, B, C ) is about 139 µm in length and about 65 µm in width. The number of teeth on the mandibles is 6, but in two individuals there were 5 and in one individual there was 7 ( Figs. 2 A, B, E ). The seta subdentalis tapers in its posterior part and reaches almost to the 5th tooth ( Fig. 2 C ). The seta interna has strong branches ( Fig. 2 E ). The mentum ( Figs. 3 A, B, C ) has a broad, rounded median tooth and between 6 and 8 apically rounded lateral teeth which decrease in size laterally. The width (W) of the mentum is presented in Table 1 . The ventromental plates (v.p.) are poorly developed and the seta submenti (s.sm) well developed and not branched ( Figs. 3A, B ). The distance (D1) between both setae submenti is provided in Table 1 . The maxilla ( Fig.3 D ) is flattened and membranous with a well-developed maxillary palp (mp), pegs and simple setae. The antenna ( Fig. 4 B ) consists of 4 segments and the lengths of segments L1 and L2 are provided in Table 1 . The fourth antennal segment is very small. The Ring Organ (RO) is on antennal segment 1. Segment 1 with a blade (bd) reaching the base of 3 rd segment. Segment 2 with style (st) reaching the apex of 3 rd segment. Lauterborn organ very weak, not easily seen. The premandible (Pm) ( Fig. 4 D ) is broad and sclerotized, with one weak apical tooth. The pecten epipharyngis (PE) ( Fig. 4 C ) consists of three parts fused at the base. The clypeus ( Fig. 4 C ) is dark, heavily sclerotized, with well-developed setae: SI, SII and SIII. Setae S1 is located close to the anterior margin ( Fig. 4 A ) and setae S2 is inserted laterally ( Fig. 4 A ). The frontal apotome is black, very well sclerotized, and elongated ( Fig. 4 A ); setae S3-S5 are located near to the lateral margin; S3 close to the anterior margin; S4, antero-medially; and S5, on median area. Near the parapod ( Fig. 4 E ) the anal setae (as) can be seen along with strong double supraanal setae (ss). Ventral tubules are absent. The pair of parapods are well developed, with heavily, dark brown sclerotized hooks ( Fig.4E ).

Comparison of the larvae of Thalassomya paraskevae and Thalassomya frauenfeldi

The larva of T. paraskevae from the Black Sea coast is distinguishable from T. frauenfeldi by a combination of morphological characters. In the larva of T. paraskevae , the median tooth of the mentum is dark and very wide ( Figs. 3A, B, C ) with 6-8 lateral teeth and in some cases different numbers of lateral teeth on each side (e.g., 8 on one side and 7 on the other). The mentum in different populations of T. frauenfeldi have 14 lateral teeth ( Wirth 1947). The mandible of T. paraskevae bears from 5 to 7 sclerotized teeth ( Figs.2 A, B, C, E ), with a very well-developed seta subdentalis reaching the 5 tooth ( Fig. 2 C ). Different populations of T. frauenfeldi all consistently have 7 teeth ( Wirth 1947). Based on the description by Wirth (1947), there are no differences between the antennae of T. paraskevae and T. frauenfeldi as the blade of the antenna of both species reaches the 3 rd segment with a noticeable RO and style ( Fig. 4B ).

Ecology of the larva

The larvae of T. paraskevae inhabit the sublittoral marine zone, which is exposed by tides. They produce tubes on stones composed of algae and sand.

Polytene chromosomes of Thalassomya paraskevae

The chromosome set is 2n = 6 plus B chromosome ( Figs. 5 A, B, C, D ). The detailed cytogenetic characteristics of as T. frauenfeldi are presented by Valkanov & Michailova (1974). Michailova (1976) described the behavior of B chromosome in meiosis of this species. In fact, the band sequences of the salivary gland chromosomes of the species presented in this study do not differ from those described by Valkanov & Michailova (1974). A characteristic feature in the karyotype of T. paraskevae in the present study is that the chromosomes are very often gathered into a ball and ectopic conjugation is observed between individual chromosomes. Most often the telomeres of the chromosomes are involved in so called an ectopic conjugation (Fig. 6 A) as are the telomeres of individual chromosomes and the B chromosome (Fig. 6 B). Here it is necessary to point out that the B chromosome is not as compact as indicated by Michailova (1989) in her study of the cytogenetic features of T. frauenfeldi . In specimens of T. paraskevae examined in the current study it is often fragmented and has a granular appearance (Fig. 6 B). Valkanov & Michailova (1974) presented a drawn map of the polytene chromosomes, and here, for the first time, the markers of each chromosome, by which the chromosomes can be identified, are indicated.

Chromosome AB ( Fig. 5A ) is metacentric. The centromere region is located in section 8. It has a dark band near to the telomere of arm A (section 1) and, respectively, a pair of dark bands in arm B (section 14) ( Fig. 5 A ) which are markers of the chromosome. After the dark band and next to the telomere of arm A, there are several compact dark bands, which are also markers of chromosome arm A (sections 1, 2, 4) ( Fig. 5A ) and not as well expressed in T. frauenfeldi as described by Valkanov & Michailova (1974). A characteristic of arm B is that the bands are significantly more compact compared to those presented by Valkanov & Michailova (1974). No chromosomal aberrations were found in this chromosome in the material examined here.

Chromosome СD ( Fig. 5 B ) is metacentric, and the centromere region is in section 8. It is characterized by an expansion at the end of arm C (section 1), surrounded on one side by a dark band, and on the other by a slight narrowing (sections 2) and a further dark band ( Fig. 5 B ). In the middle of arm C, three dark bands are found (end of the section 4 and two in section 5), surrounded by two light bands on one side and two dark bands on the other which are markers the chromosome.Another marker of this chromosome is the two dark bands located in the middle of the chromosome. One of them is the centromeric region. Only one heterozygous inversion was observed in arm D, section 13 ( Fig. 7 A ) in one individual.

Chromosome EF ( Fig. 5C ) is submetacentric and the centromere is in section 7. This chromosome carries the Balbiani Ring (BR), located in arm F (section13). Markers of the chromosome arm E is the series of light bands followed by a pair of dark bands. Not far from the telomere of arm F, there are two dark bands, between which a lighter one is located (sections 17-18) which are markers of the arm ( Fig. 5 C ). In this chromosome we found one heterozygous inversion in arm F, section 15 ( Fig. 7 B ) in a single individual.

Figures

FIGURE 2. Larva of Thalassomya paraskevae: morphology, mandible: A) mandible with 6 teeth; B) mandible with 5 teeth; C) mandible with well seen seta subdentalis; D) mouth of larva with 5 teeth of one mandible and 6 teeth in another mandible; E) mandible with 7 teeth. Scale bars, 10 µm.

FIGURE 3. Larva of Thalassomya paraskevae: morphology, mentum and maxilla:A) one side with 8 lateral teeth, the other side with 7 lateral teeth; B) one side with 5 lateral teeth, the other side with 6 lateral teeth; C) one side with 7 lateral teeth, the other side with 8 lateral teeth, well seen the place of seta submenti (s.sm) and ventromental plate (v.p), slightly developed; D) maxilla with maxillary palp (mp) and setae. Scale bars, 10 µm.

FIGURE 4. Larva of Thalassomya paraskevae: morphology, clypeus, antenna, pecten epipharyngis, premandible and caudal part: A) clypeus and frontal apotome with setae S3, S4, S5; B) antenna: blade (bl.), style (st.) and Ring Organ (RO); C) pecten epipharyngis (PE) and clypeus with setae SI, SII, SIII; D) premandible (Pm); E) caudal part with posterior parapods (PP), anal seta (as) and supra-anal seta (ss). Scale bars, 10 µm.

FIGURE 5. Polytene chromosomes of Thalassomya paraskevae:AB, CD, EF + B chromosome according to Michailova (1989): A) chromosome AB; B) chromosome CD; C) chromosome EF; D) B chromosome. Scale bars, 10 µm.

FIGURE 7. Chromosome aberrations of Thalassomya paraskevae: A) heterozygous inversion in arm D (section 13) and an ectopic pairing between arms B and F. B) heterozygous inversion in arm F (section 15). Scale bars, 10 µm.