We analyzed chromosomes of four species of fishflies (Megaloptera: Chauliodinae). Three species were from western North America (Dysmicohermes disjunctus (Walker, 1866), Dysmicohermes ingens Chandler, 1954, and Orohermes crepusculus (Chandler, 1954)), and another one from eastern North America (Nigronia serricornis (Say, 1824)). The chromosome number of the three western species was 2n = 22, with the karyotype consisting of 10 pairs of autosomes plus XY in males. The X chromosomes of these three species are subtelocentric, while the Y chromosomes are small and dot-like. Of the ten pairs of autosomes, the last pair is substantially smaller than the others. The chromosome number in the first meiotic metaphase in spermatocytes of N. serricornis from Michigan was n = 10 (9 autosomal bivalents + Xy_p in the male). The sex chromosomes of N. serricornis formed parachute-type bivalents synchronously with the autosomes. The parachute-type bivalent Xy_p has also been found in four fishflies and four dobsonflies (Megaloptera: Corydalinae) from East Asia, as well as in a fishfly and a dobsonfly from North America. These data suggest that the two subfamilies of Corydalidae share a common sex-bivalent mechanism, along with many beetles (Coleoptera).

Chromosomes, Dysmicohermes, fishflies, Nigronia, Orohermes, sex chromosomes, Xy_p/XX

The insect order Megaloptera comprises the families Sialidae (alderflies) and Corydalidae. The latter contains the two subfamilies, Corydalinae (dobsonflies) and Chauliodinae (fishflies). In North America 49 species of Megaloptera are recognized, including 24 Sialidae, 6 Corydalinae, and 19 Chauliodinae (Martins et al. 2022). The larvae of all Megaloptera are aquatic and predaceous, and they perform important functions in aquatic food webs (Rivera-Gasperin et al. 2019). Corydalinae larvae inhabit deeper waters in streams and rivers and absorb oxygen through tracheal gills on the ventral surface of their abdomens. By contrast, Chauliodinae larvae live near the banks of rivers and are primarily air breathers, using a pair of respiratory tubes on the dorsal surface of the abdomen.

Studies on Megaloptera chromosomes have been reported by Itoh (1933a, b), Hughes-Schrader (1980), and Takeuchi et al. (2002, 2012a, b). Four Japanese species of Chauliodinae had the chromosome number 2n = 20 (18+XY) (Takeuchi et al. 2012a, b) and one species from North America had the chromosome number 2n = 22 (20+XY) (Hughes-Schrader 1980). On the other hand two species of Corydalinae, from Japan and North America, both had the chromosome number 2n = 24 (22 + XY) (Hughes-Schrader 1980; Takeuchi et al. 2002).

In the present study, chromosomes of four fishfly species from the United States were studied, using preparations from larval gonads. These included three western species [Dysmicohermes disjunctus (Walker, 1866), Dysmicohermes ingens Chandler, 1954 and Orohermes crepusculus (Chandler, 1954)], and one eastern species [Nigronia serricornis (Say, 1824)] (Lacewing Digital Library 2025). Life-history information on these four fishfly species can be found in Evans (1972) and Heilveil (2004). According to data in Lacewing Digital Library (2025), there is only one species in the genus Orohermes in the world fauna, and each of the genera Dysmicohermes and Nigronia harbors two species. The objective of the current study was to characterize the chromosomes of the above four North American Chauliodinae species and to compare the results with East Asian Chauliodinae as well as several Corydalinae species. We also discuss the results in terms of insect evolutionary history.

In the present study, the chromosome number of 2n = 22 was found in the three species from western North America (10 autosomal pairs + XY in males) (Fig. 1A–C). The autosomes of O. crepusculus included four pairs of metacentric (no. 1, 5, 7, and 8), five pairs of submetacentric (no. 2, 3, 4, 6, and 9), and a single pair of small subtelocentric chromosomes (no. 10). The sex chromosomes were represented by the small subtelocentric X chromosome and the small, almost dot-like, Y chromosome (Fig. 1A). Secondary constrictions were seen near the center of the long arm in chromosomes no. 4 and 6 (Fig. 1A). The autosomes of D. disjunctus included six pairs of metacentric (no. 1, 4, 5, 6, 7, and 9), three pairs of submetacentric (no. 2, 3, and 8), and a single pair of small subtelocentric chromosomes (no. 10). The sex chromosomes were represented by the smaller subtelocentric X chromosome and the small dot-like Y chromosome (Fig. 1B). The autosomes of D. ingens included three pairs of metacentric (no. 4, 6, and 8), four pairs of submetacentric (no. 1, 2, 3, and 5) and three pairs of subtelocentric (no. 7, 9, and 10), with no. 10 being relatively small. The sex chromosomes were represented by the medium-sized subtelocentric X chromosome and the smaller dot-like subtelocentric Y chromosome (Fig. 1C). The chromosome number at the first meiotic metaphase (MI) in spermatocytes of N. serricornis was n = 10 (9 autosomal bivalents + Xy_p in the male) (Fig. 1D). During this division, sex chromosomes invariably formed parachute-type bivalents synchronously with the autosomes (Fig. 1D).

Figure 1.

Chromosomes of fishflies. Mitotic karyotypes of spermatogonial cells (A–C) and diakinesis of meiosis in the spermatocyte (D) A Orohermes crepusculus (2n = 20 + XY) B Dysmicohermes disjunctus (2n = 20 + XY) C Dysmicohermes ingens (male: 2n = 20 + XY) D Nigronia serricornis (2n = 18 + Xy_p). Arrow indicates the parachute-type sex bivalent (Xy_p). Scale bar: 5 μm.

Hughes-Shrader (1980) observed a single pair of small autosomes in the fishfly Neohermes fillicornis (Banks, 1903), which is native to western North America (Table 2). In the present study, the three western fishfly species also had one pair of small autosomes (Fig. 1A–C, Table 2). By contrast, in four Asian fishfly species, as well as in the species from Michigan, the pair of small autosomes is lacking and they all have a chromosome number of 2n = 20 (9 autosomal pairs/bivalents + Xy_p in the male) (Table 2, Takeuchi et al. 2012a, b). In addition, in five dobsonfly species from East Asia and the United States, all have a single pair of dot-like autosomes (Table 2). The presence or absence of this characteristic chromosome pair might play an important role in the karyotype evolution in the Corydalidae, which consists of Corydalinae (dobsonflies) and Chauliodinae (fishflies). In terms of the chromosome number, both East Asian and American dobsonflies have 2n = 24 (22 + Xy_p in the male), but in fishflies, 2n = 20 (18 + Xy_p in the male) in East Asian species and n = 10 (9 + Xy_p in the male) in a Michigan species (Table 2). These findings indicate that the chromosome number and karyotype structure are more variable in fishflies than in dobsonflies.

The secondary constrictions of chromosomes no. 4 and 6 in O. crepusculus (Fig. 1A) suggest that they may be the sites of rDNA clusters. In other words, the sites of rDNA clusters are observed in the secondary constrictions of chromosomes and are often located at nucleolus organizer regions (NOR). Future verification using techniques such as FISH (fluorescence in situ hybridization) or AgNOR (silver stain for nucleolus organizer region) is required (Iizuka et al. 2013; Kuznetsova et al. 2016).

In Megaloptera, the Y chromosome is dot-shaped, and when pairing occurs with the X, the XY approach each other as in autosomes, so they appear parachute-shaped. The parachute-type bivalent Xy_p was detected in N. serricornis in the present study (Fig. 1D). The parachute-type bivalent Xy_p has also been found in four other fishfly and four dobsonfly species from East Asia (Takeuchi et al. 2002, 2012a, b) (Table 2), as well as in the North American dobsonfly Corydalus cornutus Linnaeus, 1758 and fishfly N. fillicornis (Hughes-Shrader, 1980). Therefore, species of both subfamilies Corydalidae share a common sex-bivalent mechanism.

Megaloptera was once included in the order Neuroptera, but today is considered an independent order (Hayashi, 2018). The sex chromosomes in antlions and owlflies (both Neuroptera) are of the XY type (Kuznetsova et al. 2015). However, the parachute-type bivalent Xy_p has not been found in current members of the Neuroptera. The Xy_p configuration of meiotic sex chromosomes (Smith 1950) is found in many beetle (Coleoptera) families (Petitpierre 2011; Şendoğan and Alpagut-Keskin 2016; Okutaner 2020; Ruiz-Torres et al. 2021). Interestingly, recent phylogenetic analyses of DNA sequence data recognize the Coleoptera as a sister group of Megaloptera (Beutel et al. 2018). This close relationship between Megaloptera and Coleoptera is further corroborated by the presence of the parachute-type bivalent Xy_p in both orders.

We are grateful to Dr. John Applegarth (US Bureau of Land Management) for helping collect fishflies in western North America and Dr. Elwin D. Evans (Michigan Department of Natural Resources) for helping collect in Michigan. We also thank Dr. Robert A. Haack (USDA Forest Service) for contributing helpful comments on an earlier version of this paper, and Dr. Fumio Hayashi (Tokyo Metropolitan University, Japan) for providing valuable information on aquatic insects.