Research Article |
Corresponding author: David Sadílek ( sadilek11@volny.cz ) Academic editor: Snejana Grozeva
© 2016 David Sadílek, Robert B. Angus, František Šťáhlavský, Jitka Vilímová.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Sadílek D, Angus RB, Šťáhlavský F, Vilímová J (2016) Comparison of different cytogenetic methods and tissue suitability for the study of chromosomes in Cimex lectularius (Heteroptera, Cimicidae). Comparative Cytogenetics 10(4): 731-752. https://doi.org/10.3897/CompCytogen.v10i4.10681
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In the article we summarize the most common recent cytogenetic methods used in analysis of karyotypes in Heteroptera. We seek to show the pros and cons of the spreading method compared with the traditional squashing method. We discuss the suitability of gonad, midgut and embryo tissue in Cimex lectularius Linnaeus, 1758 chromosome research and production of figures of whole mitosis and meiosis, using the spreading method.
The hotplate spreading technique has many advantages in comparison with the squashing technique. Chromosomal slides prepared from the testes tissue gave the best results, tissues of eggs and midgut epithelium are not suitable. Metaphase II is the only division phase in which sex chromosomes can be clearly distinguished. Chromosome number determination is easy during metaphase I and metaphase II. Spreading of gonad tissue is a suitable method for the cytogenetic analysis of holokinetic chromosomes of C. lectularius.
holokinetic chromosomes, spreading method, squashing method, testes, midgut, karyogram
Insect chromosome research is more than 130 years old (
Historically, classical histology was the first method used for preparing arthropod chromosomes, including insect ones, when the tissue in paraffin wax was cut into sections 7-20 microns in thickness (
The most recent method “hotplate spreading” (only spreading hereinafter) was originally used only for vertebrate chromosomes studies. The whole method was then modified by
However, the use of the squashing method still strongly prevails over spreading in present Heteroptera cytogenetic studies. As the first,
One of the very frequently studied Heteroptera is the obligatorily ectoparasitic genus Cimex Linnaeus, 1758 (Cimicidae), which includes parasitologically and medically important species. This genus is characterised by possession of the all-important heteropteran cytogenetic features: holokinetic chromosomes (e.g.
In addition to the above mentioned features, the important human ectoparasite model species Cimex lectularius Linnaeus, 1758 shows intraspecific variability in number of sex chromosomes from three (X1X2Y) to 21 (X1X2Y+18 extra Xs) (e.g.
Intraspecific variability in the number of X chromosomes has been described in three cimicid species from the subfamily Cimicinae, Paracimex borneensis Usinger, 1959 (2X; 5-9X), P. capitatus (2-6X) and C. lectularius (2-20X) (summary in
Cimex lectularius became an intensively studied species by a wide spectrum of scientific approaches due to its recent massive global expansion (e.g.
The main aim of the present study is to compare results of the spreading method, used for the first time in the Cimicidae, with the traditional squashing method. We aimed to find out if the spreading method resulted in different or more conclusive data and could be therefore more suitable for analysis of cimicid holokinetic chromosomes. The use of spreading is currently quite rare even within researches of other Heteroptera species but it is also recommended for cytogenetic studies of the other insect orders. The present paper also makes comparisons of the suitability of different tissues for cytogenetic study, and of distinct cell division phases, chromosome size measurement and assembly of C. lectularius karyograms.
220 specimens of C. lectularius collected from 65 localities in 10 European countries in the period 2010–2012 were studied, for geographical origins see
The chromosome slides were examined using the Olympus Provis AX 70 light microscope and selected cells and stages of division were documented by the digital imaging system Olympus DP 72 and software QuickPHOTO CAMERA 2.3. Karyograms were made in graphic editor Corel DRAW X5. For assembly of karyograms, chromosomes were cut out from photographs, measured and sorted by size in software ImageJ 1.47 with Levan plugin (http://imagej.nih.gov/ij/).
The basic principle of the hotplate spreading technique is to turn extracted tissue into a suspension and let cells to adhere to the surface of a microscope slide (optimal is SuperFrost quality slide) as the drop was moved on the slide by pushing it with fine tungsten needles and evaporated. The resulting semipermanent slide (without cover slip) is characterized by its long durability (for years), stored at 4 °C for basic Giemsa staining or -20 °C to -80 °C for further molecular analysis (e.g. FISH). Cimex lectularius specimens were dissected in hypotonic solution 0.075 M KCl immediately after killing, to keep the gonad tissue hydrated and remove debris of other tissues. During hypotonisation, the cells receive additional water due to osmosis, making them larger, the contents of the cell are loosened and chromosomes become more individualized. Chromosomes can be damaged or washed away during final dissociation in case of excessive hypotonic treatment. However, chromosomes are still too compact and are not analysable in insufficiently hypotonised cells. Several time periods of tissue hypotonisation were tried: 10, 15, 20, 25 and 30 minutes. The best results were obtained from samples after 25 minutes of fresh hypotonic solution treatment.
Tissue fixation in methanol: glacial acetic acid 3:1 was the next step, methanol can be replaced by 99.9% ethanol. Alcohol causes immediate death of cells and acetic acid penetrates the membrane for quick ideal preservation of inner structures especially chromosomes. Two types of fixation were tested, one step fixation for 5, 10, 15 or 20 minutes, and two step fixation for 5+10, or 10+20 minutes. However, the duration of fixation had a minor effect on the final quality of chromosomes on slides. Two step fixation for 5+10 minutes was found to be optimal, the tissue dehydration effectiveness increased because in the second fixation step dilution by water from hypotonic solution was reduced to minimum.
Fixed tissue was mechanically suspended on the slide with tungsten needles and cells were chemically released by adding of 1–2 drops of 60% acetic acid. Undissociated clusters of tissue were removed. The slides with suspension were put on a warm (45 °C) histological plate and the drop was moved all around the slide with the needle. Adhering cells can create hardly diagnosable clusters without that movement. The chromosome sets are very often overlapping in those clusters. Suspension movement also contributes to evenly distributed chromosomal material on the slide surface. The slides were stained on the second day, allowing them to dry properly and to avoid loss of chromosomes. The staining was carried out using a 5% Giemsa solution in Sörensen phosphate buffer (pH = 6.8) for 10, 15, 20, 30 or 40 minutes (optimum in 30 minutes). The stained slides were stored in a refrigerator at 4 °C. The mechanism of cell adherence is described in detail by
The squashing technique is the more widely used method in Heteroptera cytogenetics. Usually, living specimens are directly fixed in ethanol: glacial acetic acid or methanol: glacial acetic acid (3:1) and can be stored at 4 °C for later use. Dissected gonad tissue is squashed under a cover slip in a drop of 45% acetic acid, which is then frozen using dry ice (solid CO2) (e.g. Kuznetsova and Nadachowska 2000, Grozeva et al. 2010,
An undoubted advantage of the squashing method is a possibility to fix material right in the field and then keep it in 70% ethanol at 4 °C for a long time (months, years), but the gonads kept longer period in cold become harder and the squashing of tissue would be more difficult. Material cannot be preserved before use of spreading method, because chromosomes from fixed cells cannot be spread. The major advantage of spreading is easier methodology. In particular, independence of dry ice or liquid nitrogen (hard to supply in the field) makes it possible to use this method outside the laboratory, with the only demand being for electricity or even without hotplate at the room temperature - higher temperature fasten the evaporation and the efficiency of the plate spreading technique.
The spreading needs manual skill in suspension droplet movement on slide after dissociation. Unsuitable manipulation could lead to loss, damage or overlap of chromosomes. On the other hand, a squashed tissue could be easily insufficiently spread and then the chromosomes on slides could be poorly, or not at all analyzable, or even the tissue can be lost during coverslip removing. The use of squashing can be very problematic in organisms with high chromosome number.
The spreading is generally an easier technique, which provides slightly better results than the squashing technique and often provides abundant slides with well-dispersed cells suitable for further analysis. Therefore, gonad tissue spreading is a suitable method for the cytogenetic analysis of Heteroptera, particularly with focus on the small, variable and numerous holokinetic chromosomes of C. lectularius. The main advantages and disadvantages of the two methods are summarised in Table
Summary of general advantages and disadvantages of the hotplate spreading and squashing methods of chromosome preparation.
Spreading | Squashing | |
---|---|---|
Material | - must be killed freshly | + can be fixed in field |
- keep it alive, store it for short time (month) | + store it for months or longer | |
Equipment | + possible to perform it in the field (need of electricity) | - not possible to perform it in the field (need of solid CO2 or liquid N) |
Overall difficulty | + lower | - higher |
+ just handle to move with droplet on slide properly with fine tungsten needles | - cells must be in chromosomes on slide is hardly analyzable single layer, if not | |
Results | + even on material rich slides is only single layer of cells | - on material rich slides is high probability of overlap |
+ i.e. more analyzable nuclei | - i.e. fewer analyzable nuclei |
Cimex lectularius reproduction is acyclic, which is why it is almost impossible to find out the exact age or physiological condition of wild specimens. Negative results from specimens with inactive gonads (absence of cell division) could be caused just by starving. Exact age and condition could be known only in laboratory reared specimens and it is also possible to use eggs or larvae of specific age.
Chromosome slides were made from tissues with the highest mitotic index, which express amount of dividing cells. Meiotic chromosomes could be isolated only from gonad tissue, but mitotic chromosomes should be obtained from all types of proliferating tissues as in insect e.g. hemolymph, epithelium of digestive tract and in holometabolous insect imaginal disc.
Gonads. Generally, tissue of gonads is used for cytogenetical studies, mainly testes (Fig.
Adult and 5th instar larva Cimex lectularius gonads. A Adult testes B Adult ovaries, without eggs C Adult ovaries, with well-developed eggs D 5th instar larva testes, well-developed, probably sub adult specimen E 5th instar larva ovaries. Scale bar = 1 mm.
Various stages of mitotic and meiotic Cimex lectularius chromosomes with basic karyotype 2n = 26+X1X2Y (A, B, D–L) and karyotype 2n = 26+X1-10Y (C), stained with Giemsa. A Mitotic prometaphase ♂ B Mitotic metaphase ♂ C Metaphase I ♂ D Leptotene ♀ E Pachytene ♀ F Diffuse stage ♂ G Diffuse stage - postpachytene transition ♂ H Postpachytene ♂ I Late postpachytene ♂ J Prometaphase I ♂ K Metaphase I ♂ L Metaphase II ♂. Arrow = sex chromosome (F, G, L) or fragments of supposedly sex chromosomes (C). Scale bar = 5 μm.
Gonads from 4th and 5th instar larvae were analyzed as well as those of adults (Fig.
In C. lectularius feeding directly initiates mating behaviour and cell division in gonads, thus this is a required condition for gonad growth and gamete production (
Testes tissues were shown to be very suitable for the C. lectularius cytogenetic research. They contain large quantities of cells in all stages of meiotic and mitotic division and provide enough information for complete karyotype analysis. Ovarian tissue is suitable in cases of lack of males or as a reference in samples with a higher chromosomal variability, and to confirm the sex chromosome system in comparison with chromosomes of males. In samples of C. lectularius with variable karyotype, it is interesting to observe complementarity of chromosome number between males and females, and it is also possible to study females with varying X chromosome numbers in oocytes, originating from breeding of specimens with different karyotypes (
The absence of meiotic metaphases in adult females suggests meiotic division in an earlier instar. However, because of quite frequently recorded pachytene nuclei (Fig.
Heteroptera cytogenetics is studied usually on male gonads. Detailed study of female karyotype is often problematic, because there is much lower abundance of dividing cells in ovaries than in testes and because all female meiotic stages are almost impossible to record. These are the main reasons for the absence of information about female cytogenetics especially meiosis (
Midgut epithelium. This tissue should be suitable for cytogenetic study due to continual wasting of digestive cells, followed by intensive mitotic division and differentiation of the regenerative (= stem) cells (e.g.
It is more complicated to obtain mitotic chromosomes from midgut epithelium than from gonad tissue in general. The use of colchicine or other mitosis-inhibiting agents, which abolish spindle formation and leave the chromosomes free in the cell, as in the studies of
Eggs. This stage of insect generally contains many tissues with a large amount of mitotic cells of the growing embryo. However, we were not successful in recognizing of these cells on spreaded slides. Three low quality mitoses were recorded on only a single slide from 13 slides analyzed. A serious complication is the unpredictable presence of eggs in wild C. lectularius females, and also the impossibility of distinguishing in advance sex of the embryos. We are sure the sex of embryos only in case of the male basic karyotype 2n = 26+X1X2Y, otherwise we are not able to distinguish between male with one more supernumerary chromosome (X1X2X3Y) and basic karyotype of female 2n = 26+X1X1X2X2.
The use of eggs is not common in Heteroptera cytogenetics, but for example in study of holokinetic chromosomes in parthenogenetic Psocoptera (
The karyotype was successfully determined in 128 out of 220 specimens of C. lectularius (58%), 80 males and 48 females, from 140 positive chromosomal slides (34%) (with cells in division) out of 412 examined. Slides prepared from testes tissue gave the best results, 90 positive slides out of 170 (53%). Ovarian tissue contains only mitosis with a lower number of 50 positive slides out of 111 (45%). However, the tissues of midgut and eggs were surprisingly unsuccessful, with only 2 positive slides out of 125 (1.6%). All slides were treated identically, therefore a ratio between positive and negative slides could show percentage of specimens in ideal physiological state for getting mitotic and meiotic chromosomes.
The following stages of cell division were observed with various frequencies in C. lectularius males. Mitotic cells were recorded especially in metaphase and prometaphase stages (Fig.
On slides from ovary cells in mitotic metaphase stage (100% of specimens) only early prophase I (leptotene and pachytene) (Figs
Leptotene (Fig.
In metaphase I (Fig.
Chromosome number determination is notably easier in meiotic metaphase I and II than in mitosis, because chromosomes are paired and superspiralized. These results show that, using the spreading method, it is possible to get mitotic and meiotic chromosome slides in high quality for further analysis.
Well-spread mitotic chromosomes can be used to assemble karyograms. A particular requirement here is that chromosomes are not physically stretched in the course of preparations, as can happen with squashes. A karyogram represents standard format of species karyotype image that helps us to distinguish chromosomes, generally specific pairs of autosomes and sex chromosomes (e.g.
We assembled three examples of karyograms from C. lectularius mitotic chromosomes from different chromosome number of 2n = 29, 33 and 37 (Fig.
Male mitotic and meiotic karyograms of Cimex lectularius chromosome variants. A-C Mitotic prometaphase. A 2n = 26+X1X2Y B 2n = 26+X1-6Y C 2n = 26+X1-10Y D Prometaphase II, 2n = 26+X1X2Y E Metaphase II, 2n = 26+X1-7Y. Scale bar = 5 µm.
In each of C. lectularius karyotypes the size of chromosomes gradually decreases. That is a reason why the size expressed only as a percentage is not very suitable for karyotype comparison among congeneric species with different diploid chromosome numbers because they have different distribution of length. However, in case of C. lectularius chromosome fragments we can predict their very small size as on example of metaphase I with 2n = 26+X1-10Y (Fig.
The hotplate spreading technique has many advantages in comparison with the squashing technique. It is suitable for use by cytogenetic beginners as they need only to get the manual skill in suspension droplet movement on slide. One disadvantage of spreading exists – material has to be prepared freshly after killing, either in the field or after keeping alive in a lab. However, C. lectularius is capable to survive in good health several months without any meal. The spreading technique seems to be ideal for study of specimens with numerous holokinetic chromosomes.
Tissue of testes, the usual material for insect cytogenetic studies, appeared to be the most suitable also in chromosome study of C. lectularius. Ovaries sometimes also show some interesting results. But the tissue of midgut and eggs – supposedly suitable, did not show any satisfactory results.
Results based on ovarian tissue could be easily misinterpreted. During dissociation, cells from ovaries and developing male and female embryos resulted from mating with unknown karyotype male could be mixed. Thus it is possible to observe artificial heterogenic sample of three karyotypes, which can be misleadingly considered as a variability in one female karyotype. This is made possible thanks to cimicid specific traumatic insemination and egg fertilization directly in ovarioles, whole effect could be also magnified by low abundance of mitotic nuclei in ovarian tissue in general. To avoid this problem would be necessary to separate only germarium, part of ovaries where mitosis give rise to primary oocytes.
Meiotic metaphase II is the best division phase for study of chromosomes in C. lectularius, but mitotic prometaphase and metaphase I are also usable. Our suggestion that the abundant nuclei in diffuse stage could serve for quick diagnosis of sex chromosome number was not proved. Nuclei of specimens with higher number of sex chromosomes did not show clear number of heteropycnotic sex chromosome elements. The explanation could be either that spiralization of sex chromosome fragments has changed so they are no more positively heteropycnotic during diffuse stage, or too small size of fragments.
We would like to express special thanks to María José Bressa for valuable advice. We are greatly obliged to all specialists who helped us with sampling of bed bugs: A. Drozda, P. Dvořák, P. Foltán, S. Kováč, M. Kučera, V. Měřínský, M. Novotný, L. Pěček, V. Prchal, S. Ritterová, P. Sodomek, R. Šimák, M. Toman, J. Vondráčková and J. Zelená (Czech Republic); L. Duplantier (France), T. Hutson (Great Britain), S. Boscolo (Italy), Ł. Brożek, M. Kadej, P. Szewczyk (Poland), H. Kjellberg, A. Larson (Sweden), E. Krug, M. Schmidt, M. Wegman (Switzerland). We appreciate also great help of pest control managers from Biotech Salzburg (Austria), DDD servis (Czech Republic), Ekolas (Slovakia), Pelias Norsk Skadedyrkontroll (Norway) and Verminex (Poland). We are also grateful to O. Balvín (Czech University of Life Sciences, Prague) for help with material collecting and M. Forman and J. Hadrava (Charles University, Prague) for valuable comments to the manuscript.
This project was supported by the grants no. 267111/2011 and 277815/2015 of the Grant Agency of Charles University and grant of Ministry of Education, Youth and Sports of the Czech Republic no. SVV 260313/2016 to senior author.