Two different forms of cryptic species-complexes in mites of the Histiostomatidae (Astigmata) from bank mud and bark beetle-galleries and their significance for applied biodiversity research

von wirthstef

Chapter 1

Bark beetle-associated cryptic mite species

The species Histiostoma piceae Scheucher, 1957, H. ulmi Scheucher, 1957 and H. scheucherae Scheucher, 1957 (described as H. trichophorum (non Oudemans), renamed by Mahunka in 1963 into H. scheucheri and nom. emend. by Klimov & Khaustov in 2018 into H. scheucherae) are morphologically similar in terms of deutonymphs and adults (Fig. 1 E, F). I mentioned in Wirth & Pernek (2012) that they share morphological characters of females. I herewith state that they based on their morphology in all instars (deutonymphs, adulti) presumably form an own cryptic clade within the much more species-rich monophyletic Histiostoma piceae-group. In Wirth & Pernek (2012) I furthermore assumed H. piceae and H. ulmi representing sister species by sharing more morphological characters with each other than with H. scheucherae. Morphological arguments for a monophyletic cryptic complex of these three species are presented below. It might in further investigations come out that this cryptic clade is even bigger.

Phoretic hosts

According to Scheucher (1957), H. piceae is host specific for Ips typographus (Scolytinae). With the colleagues M. Pernek and O. Weis (2016) I concluded from material, collected in Croatia in 2012, that Ips cembrae (sister species of I. typographus) additionally regularly carried H. piceae.
H. ulmi was described by Scheucher (1957) as being phoretically found on Hypophloeus bicolor that appeared in the galleries of Scolytus sp.. Thus an indirect association with Scolytus sp. is also assumed, but according to Scheucher without attaching Scolytus itself.
I determined H. ulmi based on all developmental in stars (deutonymphs and adults) from galleries of the cerambycid beetle Acanthocinus reticulatus that had infested the bark of an Abies alba tree in Croatia (Wirth & Pernek, 2012). Although mites were isolated from bark samples, the direct phoretic association with A. reticulatus was stated, as under the controlled laboratory conditions no other inhabitants of this wooden block existed. In comparable blocks from the same felled tree at different heights additionally the beetles Pityokteines spinidens and P. curvidens appeared as wood infection pioneers. H. ulmi was not found in their galleries during these studies, but in Wirth & Pernek (2012) we discussed the option that some deutonymph samples from these beetles from former studies, determined as H. piceae (Pernek er al., 2008), might represent in parts or entirely H. ulmi.

H. scheucherae was collected by Scheucher (1957) from sapflux of a birch and a beech, additionally on Helops quisquilius and Hypophloeus bicolor (both Tenebrionidae).

Fig. 1: A = ventral deutonymph and B =  dorsal adult female of Histiostoma piceae; C = ventral deutonymph and D = dorsal adult female of Histiostoma ulmi; E = ventral deutonymph and F = dorsal adult female of Histiostoma scheucherae; d.f. = digitus fixus ending, sc i to h1 = dorsal setation of female; drawings of Scheucher (1957) newly arranged and modified by adding captions.

Morphological similarities and differences

a) Similarities

According to Scheucher (1957), the deutonymphs of all three species are more or less the same size (median lengths from the anterior Gnathosoma to the posterior hysterosoma around 200 µm), the sizes of the adults vary with regard to the females (lengths measured as above between 300 and 400 µm). However, it must be taken into account that females show cuticle growth with increasing egg maturation (e.g. Wirth 2004).

The deutonymphs of the three species are confusingly similar. Distinct differences are difficult to find (Figs. 1 A, C, E, undersides of the deutonymphs). The dorsal setae are long and point forward (not shown, see for example in Wirth, 2004).

On the ventral side of the deutonymphs, the apodeme anterior of the second legs is conspicuously shaped: the apodeme rises laterally anteriorly and then forms a wide angle with which it slopes slightly towards the median area of the body. This creates a structure that is reminiscent of an elongated „M“.

The two apodemes, which arise anterior to the pair of legs I and II, end free and do not touch the „M“ apodeme (Figs. 1 A, C, E).

The adult females show three humps on each side of the dorsal hysterosoma, which represent bulges of the cuticle. The humps are homologous to one another, which can be seen, for example, from the fact that they share corresponding setae (Figs. 1 B, D, F): anterior setae c1-c3, in the middle setae e1 and d2 and posteriorsetae h1 (labeled in Fig. 1B).

The distal end of the digitus mobilis, which consists of 2 to 3 „teeth“ and has the same design in males and females, is also systematically relevant (Figs. 1 B, D, F each at the top left in the picture). In all species, the palpar membrane has an additional fold on both sides of the exit opening of the chelicerae (not shown here, see Scheucher, 1957 or Wirth, 2004).

According to Scheucher (1957), the males of the three species are significantly smaller than the females and, especially in H. piceae and H. ulmi, not significantly larger than the deutonymphs (in all three species 200-300 micrometers, measured as above). Their dorsal sides are poorly described and not shown in Scheucher (1957).

What they have in common is that the anterior apodeme of the second pair of legs tends to touch the same apodeme on the opposite side medianly. The four ring organs are round in all three species, and thus possess roughly the same shape and arrangement (male not shown here, see Scheucher, 1957).

b) Morphological differences based on Scheucher (1957)

The dorsal setae of the deutonymphs of H. piceae appear thicker and stronger than those of the other species (not shown here, see Scheucher (1957). The ventral apodeme anterior of the third legs runs straighter in the middle part in H. scheucherae unlike the other two species and therefore gives only a reduced idea of ​​the „M“ character (Fig. 1 E).

The dorsal humps on the hysterosoma of the females are steep in H. piceae, almost even pointed, and in H. scheucherae and H. ulmi flatter and more rounded (Figs. 1 B, D, F). On the dorsal proterosoma, the seta sc e in H. piceae is on a raised cuticle bulge, in H. ulmi this bulge is flatter, in H. scheucherae it seems to be absent or only hinted at (Figs. 1 B, D, F).

The digitus fixus end of the chelicera has 3 flat teeth in H. piceae, 3 clearly defined teeth in H. ulmi and only two teeth in H. scheucherae (Figs. 1 B, D, F, at the top left in the picture).

In the male, the ventral apodeme in front of the second legs is fused with the corresponding opposite apodeme, in H. ulmi the two apodemes only almost touch and in H. scheucherae they also nearly touch, but then bulge slightly backwards (not shown here, see Scheucher, 1957).

c) Discussion of phylogenetic conclusions

As strong features to establish a monophyly of H. piceae, H. ulmi and H. scheucherae, I only consider the humps on the hysterosoma of the females. The forward-facing dorsal setation of the deutonymphs is possibly a feature of all members of the bark-beetle clade. The more or less M-shaped apodeme of the deutonymph too. The distribution of the distal cheliceral trait in adult mites is also difficult to assess. It may not be suitable as an apomorphy of the above named species. The palpar membrane feature applies to the entire bark-beetle clade. However, the development of an elevation on the dorsal proterosoma of the females, on which the setae sc e arise, may be an indication of a sistergroup relationship between H. piceae and H. ulmi. Another feature mentioned in Wirth & Pernek (2012) on the ventral palposoma of the deutonymph to justify this sistergroup relationship meanwhile seems to me as too weak.

d) Discussion of determination problems

It is not a concern that we in Wirth et al. (2016) identified the bark beetle Ips cembrae as an additional host for H. piceae, as it is the closest relative of I. typographus that Scheucher named as the sole host. However, H. piceae is also referred to as the phoretic companion of various other beetle species, for example in Pernek et al. (2008) on Pityokteines species. As is the case here, mites in such publications have often only been determined according to the deutonymph. Either the assumed host specificity does not apply or such finds have been confused with the other species mentioned here, or they represent still unknown and very similar species. A possible confusion of the findings in the above-cited publication with H. ulmi is discussed in Wirth & Pernek (2012) alongside other hypotheses.

In 2002 I collected H. piceae individuals from a block of spruce wood at the Free University of Berlin with Ips typographus and took SEM photos. The mites corresponded to the species description of Scheucher (1957) in all stages and details, but a trend became apparent, according to which the distal digitus fixus contained more clearly defined 3 teeth than seen by Scheucher (not shown here, never published before, part of a recent still unpublished project).

These teeth of the distal chelicera, made visible by scanning electron microscopy, resemble those of H. ulmi. Since the difference in the chelicerae morphology is the strongest feature to distinguish these two species, questions arise about the general variability within populations of the same species of the features listed in this chapter.

I identified H. ulmi from tunnels of Acanthocinus reticulatus (Wirth & Pernek, 2012). An association with Pityokteines species is also discussed in the same paper. In addition, the ventral apodeme of the males is shown via a light microscope, which only almost touches the corresponding opposite apodeme medianly and then curves backwards slightly (illustration in Wirth & Pernek, 2012). This feature resembles the homologous apodeme of Histiostoma scheucherae and differs only slightly from the Scheucher drawing of H. ulmi. Nevertheless, the mite bred in Croatia at that time corresponds in all other relevant characteristics to the H. ulmi description by Scheucher (1957).
Due to different hosts and a slightly different morphology, the question should be asked, for example, whether the Croatian mite (Wirth & Pernek, 2012) could possibly be an independent, as yet unknown species, which then would belong to the cryptic species complex of H. piceae, H. ulmi and H. scheucherae or whether even a subspecies status of H. piceae is possible.

With regard to the three species from bark beetle substrate discussed here, it can be stated that despite the knowledge of deutonymphs and adults, it is difficult to distinguish the species from one another. If only the deutonymphs were used for these species due to faster workflows, clear identifiability would be so significantly restricted that incorrect determinations would be possible or even probable. More precise differentiation at such a level can best be achieved through DNA barcoding.

In the case of the presence of all stages as well as living cultures in other mites of the Histiostomatidae, various possibilities have basically already been used in order to obtain additional characteristics for a better characterization of the species. The use of high-resolution optics, such as scanning electron microscopy (e.g. Wirth, 2005) or CT technology (Dunlop et al., 2011), can help to find and compare additional morphological character complexes, which would be difficult to analyze with means of the conventional lightmicroscopy. For this purpose, at least two, better still three, development stages should be available in order to increase the number of potentially meaningful features. An example of this approach can be found in the second chapter of this publication.

The combination of ultrastructural studies (TEM) and biological findings might for example contribute to the discovery that for example several very similar species had been mistaken with just one. An example is the mite Histiostoma feroniarum, which does not belong to the bark beetle mites, but to the relationship of the species mentioned in the second chapter. It is one of the most common mites of the Histiostomatidae and is said to be found on countless arthropods in a wide variety of habitats. I assume that only those populations that reproduce arrhenotokously and show a sexual dimorphism of the males actually belong to this species (Wirth, 2005B). Witalinski et al. (2014) examined two culture strains that were originally attributed to H. feroniarum, ultrastructurally: one reproduced itself arrhenotokously, the other thelytokously. The authors found that gonadal morphology and egg development differ distinctly, which is why two species could be distinguished, of which only the former represented H. feroniarum.

The (at least according to Scheucher, 1957) differently designed chelicerae in each of the species H. piceae, H. ulmi and H. scheucherae indicate different modalities for ingestion. It is also possible that the food itself is different. Identifying the exact food composition might aid in a well-founded identification of cryptic species. However, it is not yet fully known which foods Histiostomatidae incorporate. This can also be different in different taxa of the Histiostomatidae. What all species have in common, however, is that the gnathosoma is transformed into a filter-feeding organ (e.g. Wirth, 2006).

Studies on a few species of mites of the Histiostomatidae suggest a preference for fungi. According to Bahrami et al. (2012) feeds Histiostoma feroniarum on Fusarium graminearum, while Yousuf et al. (2014) suggest that the bark beetle-associated mite Histiostoma varium might feed on the blue stain fungus Ophiostoma ips. Klimov and Khaustov (2018) point out that various bark beetle-associated mites of the Histiostomatidae, through their phoretic deutonymphs, are not only carrying fungal spores of numerous species of fungi, but also act as vectors for those fungi that ultimately grow in the bark beetle galleries, where some serve as a food source for the mites. However, it is still unclear, whether the filter-feeding apparatus of the Histiostomatidae is generally only specialized incorporating fungal hyphes. The term „microorganisms“ has also been used on various occasions to denote the nutritional basis of these mites. In fact, for the species Bonomoia opuntiae, I observed that flakes were incorporated in an aqueous medium consisting of bacterial colonies, micro-algae and perhaps also fungi (Wirth, 2005).

Studies on the exact composition of the diet do not yet exist for the species H. piceae, H. ulmi and H. scheucherae. Such studies could help differentiate these species and their closest relatives more clearly from one another.

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