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Brazilian Journal of Biology

Print version ISSN 1519-6984On-line version ISSN 1678-4375

Braz. J. Biol. vol.75 no.4 São Carlos Nov. 2015  Epub Nov 24, 2015

http://dx.doi.org/10.1590/1519-6984.04614 

Articles

ITS-2 sequences-based identification of Trichogramma species in South America

Identificação molecular de espécies de Trichogramma na América do Sul via sequenciamento da região ITS-2

R. P. Almeidaa  * 

R. Stouthamerb 

aCentro Nacional de Pesquisa de Algodão, Empresa Brasileira de Pesquisa Agropecuária – EMBRAPA Algodão, CP 174, CEP 58428-095, Campina Grande, PB, Brazil

bDepartment of Entomology, University of California, Riverside, CA 92521, USA


Abstract

ITS2 (Internal transcribed spacer 2) sequences have been used in systematic studies and proved to be useful in providing a reliable identification of Trichogramma species. DNAr sequences ranged in size from 379 to 632 bp. In eleven T. pretiosum lines Wolbachia-induced parthenogenesis was found for the first time. These thelytokous lines were collected in Peru (9), Colombia (1) and USA (1). A dichotomous key for species identification was built based on the size of the ITS2 PCR product and restriction analysis using three endonucleases (EcoRI, MseI and MaeI). This molecular technique was successfully used to distinguish among seventeen native/introduced Trichogramma species collected in South America.

Keywords:  molecular markers; ITS-2 sequence; Hymenoptera; egg parasitoid

Resumo

Sequências do Espaço Transcrito Interno 2 (ITS2) têm sido utilizadas em estudos taxonômicos e sua utilidade constatada pela confiabilidade que o método confere à identificação das espécies de Trichogramma. Esta técnica molecular foi bem sucedida em distinguir dezessete espécies nativas e introduzidas de Trichogramma, coletadas na América do Sul. As sequências do DNAr variaram de 379 a 632 pb. Em 11 linhagens de T. pretiosum estudadas, o endosinbionte Wolbachia foi detectado pela primeira vez. Estas linhagens telítocas foram encontradas no Peru (9), Colômbia (1) e Estados Unidos (1). Uma chave dicotômica para identificação de espécies foi construída baseada no tamanho do produto da PCR do ITS2 e em análises de restrição utilizando-se três endonucleases (EcoRI, MseI and MaeI).

Palavras-chave:  marcadores moleculares; sequências ITS-2; Hymenoptera; parasitóide de ovos

1 Introduction

Natural enemies of the family Trichogrammatidae are released worldwide against a number of lepidopterous pest on corn, rice, sugar-cane, cotton, vegetables and pines (Smith, 1994; Li, 1994; Grenier, 1994). In the past, most successful biological control agents have been found through trial and error. Nowadays, there is an effort to predict the success of a Trichogramma strain before introductions are made (van Lenteren and Woets, 1988).

Therefore, standard procedures to compare Trichogramma candidates for biological control have been proposed (Hassan, 1994). According to Hassan (1995) before field releases are undertaken, a suitable Trichogramma strain of known qualities should be chosen. The effectiveness of Trichogramma in the field largely depends on its searching behaviour, host preference and tolerance to environmental conditions. However, correct identification of the Trichogramma species that is going to be tested, is a crucial step in any biological control program.

Despite the large improvement with the discovery of male genitalic morphology as species specific characters (Nagarkatti and Nagaraja, 1968, 1971), the identification of these tiny insects remains difficult and requires specialized skills (Pinto and Stouthamer, 1994).

Their small size and lack of morphologically distinct characters has historically been a problem in taxonomic studies. Specimen must be prepared on slides for examination, a time consuming process that requires considerable experience (Platner et al., 1999).

Unfortunately many important species share similar genitalic structures and this has forced workers to continue relying on less dependable characters that often are intraspecifically variable and subject to phenotypic plasticity (Pinto et al., 1989; Pinto and Stouthamer, 1994).

To simplify the Trichogramma species identification, several methods have been proposed: (1) Allozymic analysis (Pintureau and Babault, 1980, 1981, 1982; Pintureau and Keita, 1989; Kazmer, 1991; Pinto et al., 1992, 1993; Pintureau, 1993). Esterase electrophoresis has provided consistent results for differentiation of some Trichogramma species (Pinto et al., 1992, 1993; Silva et al., 1999). The limited variation at esterase loci only allows for the differentiation between a limited number of species (Richardson et al., 1986; Pinto and Stouthamer, 1994). In addition the samples subjected to allozyme electrophoresis have to be kept at –70 °C to avoid the degeneration of the enzymes; (2) Reproductive compatibility tests (Nagarkatti and Nagaraja, 1968; Pinto et al., 1991; Pintureau, 1991) in which individuals of the unknown species are crossed with individuals of known species status; (3) The DNA sequence of ribosomal spacers (Landry et al., 1993; Orrego and Agudelo-Silva, 1993; Sappal et al., 1995; van Kan et al., 1996, 1997; Pinto et al., 1997); (4) RFLP’s of complete mitochondrial genome has also been used for species differentiation (Vanlerberghe-Masutti, 1994). Ribosomal DNA is present in all organisms and is composed of several regions (genes and spacers) that evolve at different rates (Hillis and Dixon, 1991); (5) The internal transcribed spacer regions (ITS-1 and ITS-2) have been used at species and intraspecific levels of many taxa for distinguishing the different forms (Carbone and Kohn, 1993; Bowles and McManus, 1993; Hsiao et al., 1994; Buckler IV et al., 1997). In insects these sequences have been often used for taxonomic purposes (Campbell et al., 1993; Hoy, 1994; Kuperus and Chapco, 1994; Vogler and DeSalle, 1994; Stouthamer et al., 1999; Silva et al., 1999; Chang et al., 2001; Pinto et al., 2002; Honda et al., 2006; Dem'yanchuk et al., 2008; Kumar et al., 2009; Polaszek et al., 2011; Poorjavad et al., 2012).

Here, we use the ITS2 sequences of Trichogramma species (native or introduced) from South America to develop an identification key. The advantage of this identification system over the morphology-based system is that non-specialists are able to quickly and cheaply identify individual specimens. In addition females can also be identified which is not possible in the morphologically based system. Species found in South America are thelytokous, either because of infection with parthenogenesis inducing Wolbachia (T. pretiosum and T. atopovirilia) (Grenier et al., 1998; Almeida et al., 2001, 2010; Ciociola Junior et al., 2001a) or because of some nuclear genetic factor (T. cacoeciae Marchal) (Stouthamer et al., 1990; Almeida and Stouthamer, 2003). Variation of ITS2 sequence within Trichogramma species is relatively small in comparison to the difference found between species and all morphologically distinct cryptic species are also distinguished by sequence differences (Stouthamer et al., 1999). ITS2 sequences within each species are very similar and there is no evidence for two or more gene families that differ substantially within the genome of a single individual, as has been found in other taxa (Vogler and DeSalle, 1994).

2 Material and Methods

2.1 Trichogramma cultures

Table 1 lists the Trichogramma species/lines studied here, including their origin, host insect, plant crop, collection date and status as native or introduced.

Table 1 Trichogramma populations from South America. 

Spécies Line Designation Origin Host Insect Plant Crop Collection Date
T. acacioi Brun, Moraes and Soares, 1984 Taca-01 Botucatu-SP-Brazil Euselasia sp. - -
T. atopovirilia Oatman and Platner, 1983 Tato-01 Sete Lagoas-MG-Brazil - Corn -
Tato-02 Colombia - - -
T. brassicae Bezdenko, 1968 Tbra-01 Introduced from Germany - - Dec., 1996
T. bruni Nagaraja, 1983 Tbru-01 Piracicaba-SP-Brazil Heliconius phyllis Passiflora sp. -
T. cacoeciae Marchal, 1927 Tcac-01 Huarochiri-Peru Cydia pomonela Apple Oct., 1997
T. dendrolimi Matsumura, 1926 Tden-01 Introduced from Germany - - Dec., 1996
T. esalqueanumQuerino and Zucchi, 2003 Tesa-11 Piracicaba-SP-Brazil Helicornius erato phyllis Passiflora sp. Oct., 1999
T. exiguum Pinto and Platner, 1983 Texi-01 Peru (different places) Diatraea saccharalis Sugar-cane, -
Helicoverpa zea Corn -
Erynis ello Cassava -
P. persimilis Olive -
Dione juno Passiflora P. Mar., 2001*
Texi-02 Colombia - - -
T. fuentesi Torre, 1980 Tfue-01 Peru Diatraea saccharalis Sugar-cane, rice -
Helicoverpa zea Corn -
Heliothis virescens, Anomis texana Cotton Dec., 1994*
T. galloi Zucchi, 1988 Tgal-01 Chiclayo-Peru Diatraea saccharalis Sugar-cane Apr., 2000
T. iracildaeQuerino and Zucchi, 2003 Tira-01 Maceió-AL-Brazil Calpodes ethlius Canna spp. Apr., 2000
T. lasallei Pinto, 1998 Tlas-01 Lima-Peru Quinta cannae Canna indica Mar., 2001
T. lopezandinesis Sarmiento, 1983 Tlop-01 Colombia - - -
T. nerudai Pintureau and Gerding, 1999 Tner-01 Chile Rhyacionia buolina Pine forest -
T. pintoi Voegelé, 1982 Tpin-01 Introduction from Chile - - Jun., 1973
T. pretiosum Riley, 1879 Tpre-01 Chincha-Peru Diaphania nitidalis Pumpkins Mar., 1992
Tpre-02 Chancay-Peru Dione juno Passiflora May, 1994
Tpre-03 Lambayeque-Peru Diatraea saccharalis Corn Jul., 1994
Tpre-04 Tacna-Peru Palpita persimilis Olive Apr., 1995
Tpre-05 Moquengua-Peru Marasmia trapezalis Corn Jul., 1996
Tpre-06 Ica-Peru Heliothis virescens Cotton Nov., 1996
Tpre-07 Piura-Peru Alabama argillacea Cotton Apr., 1997
Tpre-08 Lima-Peru Diaphania nitidalis Curcubita moschata Mar., 1997
Tpre-09 Ayacucho-Peru Helicoverpa zea Corn Nov., 1998
Tpre-10 Colombia Neoleucinodes elegantalis - -
Tpre-11 USA - - -
Tpre-12 Jataizinho-PR-Brazil - Cotton Feb., 2000
T. rojasi Nagaraja and Nagarkatti, 1973 Troj-01 Curitiba-PR-Brazil Anticarsia gemmatalis - -

*Last collection date.

2.2 DNA Extraction, PCR Amplification and electrophoresis

To extract DNA from the different species/lines we used five wasps that were homogenized in 100 µL 5% Chelex-100 and 4 µL proteinase K (20 mg/mL) and incubated for at least 4 hours at 56 °C, followed by 10 min. at 95 °C. The PCR was performed in a total volume of 50 µL using a Techne thermocycler, 5 µL DNA template, 5 µL 10x PCR-buffer, 1 µL dNTP’s (each in a 10 mM concentration), 1 µL forward and reverse primers (ITS2-forward: 5’ -TGTGAACTGCAG GACACATG-3’ located in the 5.8S region of the rDNA; ITS2-reverse: 5’ -GTCTTGCC TGCTCTGCTCTGAG-3’ located in the 28S region of the rDNA (Stouthamer et al., 1999); 0.14 µL TAQ polymerase (5 units/µL) and 36.86 µL of sterile distilled water. The cycling program was 3 min. at 94 °C followed by 33 cycles of 40 seconds at 94 °C, 45 second at 53 °C and 45 seconds at 72 °C with 5 min. at 72 °C after the last cycle.

2.3 Cloning, sequencing and alignments

Following electrophoresis, PCR products were purified with a QIAquick PCR purification kit (Qiagen®). After the purification the PCR products were ligated into a Pgem-T®Vector (Promega), 2 µL of the ligation mix was transformed in the heat shock cells of DH5-α Escherichia coli and plated on a LB agar medium containing Ampicilin, XGAL and IPTG. The plates were incubated overnight at 37 °C. The next day, white colonies were picked up with a sterile toothpick from the plates and placed into tubes containing 3.0 mL of LB liquid medium and 3 µL Ampicilin and put to grow up overnight in a shaker set to 250 rpm at 37 °C. To confirm that the correct piece of DNA had been cloned, a PCR reaction with a template extracted from the bacterial culture was added to 100 µL 5%

Chelex-100 and incubated for 15 min. at 60 °C followed by 5 min. at 95 °C. The PCR was performed with 5 µL of this template as described before. If indeed an ITS2 had been cloned, 850 µL of the bacteria culture was added to 150 µL of 87% glycerol and stored at –80 °C. The rest of the culture was used in a QIAprep Miniprep kit (Qiagen®) to purify the plasmid, which was used for the sequencing in an Applied Biosystems automatic sequencer. Trichogramma sequences were aligned manually using the ESEE 3.0s sequence editor (Cabot, 1995).

2.4 Thelytoky in Trichogramma

For detection of thelytoky caused by Wolbachia infection, specific primers for DNA amplification of the wsp region of Wolbachia were used: wsp-Forward primer 5’TGGTCCAATAAGTGATGAAGAAAC-3’ and wsp-Reverse 5’ –AAAAATTAAACGCTACTC CA-3’ (Braig et al., 1998). These primers amplify 554bp of the wsp gene. The cycling program was 3 min. at 94 °C followed by 40 cycles of 1 min. at 94 °C, 1 min. at 50 °C and 1 min. at 72 °C with 5 min. at 72 °C after the last cycle.

2.5 Molecular key

A molecular key for separating Trichogramma species was constructed based on the size and the fragment profile following restriction with different restriction enzymes. Initially we determined ITS2 sequences of all species and lines, subsequently we first tried to distinguish the different species based on the size of the PCR product. Species with similar sized PCR products were distinguished by selecting restriction enzymes that would generate differently sized in restriction fragments. To predict the fragment sizes to be expected with different enzymes we use the program Webcutter 2.0 (Heiman, 1997). Three restriction enzymes were used (EcoRI, MseI and MaeI).

3 Results

Seventeen native and introduced Trichogramma species collected in South America were identified based on ITS2 sequences. In total, thirty lines were studied (Table 1). Seventeen lines were recognised as arrhenotokous and thirteen as thelytokous. Twelve out of thirteen thelytokous lines studied here carry the Wolbachia symbiont. In T. cacoeciae, parthenogenesis was not caused by Wolbachia infection (Stouthamer et al., 1990; Almeida and Stouthamer, 2003). For all other parthenogenetic lines, Wolbachia detection was possible by using specific primers of the wsp region. In all thelytokous species/lines no male was found, except for T. atopovirilia (culture form Brazil) and T. pretiosum (culture from the Entomology Lab., California University, Riverside-USA) in which few males were present.

Complete ITS2 sequences have been deposited in GenBank (Table 2). Trichogramma species were identified by (1) morphological identification: T. atopovirilia, line Tato-01, (Dr. Américo I. Ciociola Junior - EPAMIG, Brazil, personal comunication), T. lopezandinensis and T. fuentesi (Dr. Ranyse B.Q. da Silva-ESALQ/USP, Brazil, personal communication); (2) by comparison with ITS2 sequences from the GenBank: T. pintoi, T. lasallei, T. cacoeciae, T. galloi, T. pretiosum, T. brassicae, T. dendrolimi and T. exiguum; or (3) by comparison with sequences not published yet: T. nerudai. T. rojasi and T. acacioi lines were the same as those used by Ciociola Junior et al. (2001b). Two new species from Brazil (T. iracildae and T. esalqueanum) sequenced here were recorded by Querino and Zucchi (2003).

Table 2 Line designation, reproduction mode, genbank accession number of the Trichogramma species and size of the ITS2 and PCR product (bp). 

Line Designation Reproduction Mode Acession Number ITS-2 Product PCR product
Tner-01 Arrhenotoky AY182756 632 746
Tpin-01 Arrhenotoky AY182757 581 695
Tato-01 Thelytoky AY182758 565 680
Tato-02 Arrhenotoky AY182759 561 675
Taca-01 Arrhenotoky - 559 674
Tbru-01 Arrhenotoky AY187263 536 650
Troj-01 Arrhenotoky - 524 638
Tira-01 Arrhenotoky AY182760 522 636
Tlop-01 Arrhenotoky AY182761 509 623
Tlas-01 Arrhenotoky AY182762 485 599
Tesa-11 Arrhenotoky AY182763 379 493
Tcac-01 Thelytoky AY166700 460 574
Tgal-01 Arrhenotoky AY182764 445 560
Tfue-01 Arrhenotoky AY182765 437 553
Tpre-01 Thelytoky AY182770 410 524
Tpre-02 Thelytoky AY182771 410 524
Tpre-03 Thelytoky AY182772 415 529
Tpre-04 Thelytoky AY182773 410 524
Tpre-05 Thelytoky AY184958 410 524
Tpre-06 Thelytoky AY184959 410 524
Tpre-07 Thelytoky AY187259 415 529
Tpre-08 Thelytoky AY184960 410 524
Tpre-09 Thelytoky AY184961 410 524
Tpre-10 Thelytoky AY187260 412 526
Tpre-11 Thelytoky AY187261 413 527
Tpre-12 Arrhenotoky AY187262 412 526
Tbra-01 Arrhenotoky AY182766 406 520
Tden-01 Arrhenotoky AY182767 403 519
Texi-01 Arrhenotoky AY182768 383 497
Texi-02 Arrhenotoky AY182769 381 496

An identification key was constructed for these species using as two characters of the PCR product its size and the restriction patterns generation using the three restriction enzymes EcoRI, MseI and MaeI (Table 3). España-Luna et al. (2008) used two restricion enzymes (EcoRI and AluI) for distinguishing six Trichogramma species from each other in Mexico. Sumer et al. (2009) used three restricion enzymes (MnlI, MseI and DraI) for distinguishing six Trichogramma species from each other in agricultural settings around the Mediterranean.

Table 3 Molecular key for Trichogramma species recognition based on the size of the PCR product and species-specific banding pattern. 

1. Size of the PCR product < 620 bp 2
Size of the PCR product > 620 bp 10
2. Size of the PCR product < 550 bp 3
Size of the PCR product >550 bp 7
3. Size of the PCR product ≤ 500 bp 4
Size of the PCR product > 550 bp 5
4. PCR product cut by MseI ca. 280 and 191bp T. esalqueanum
PCR product not cut by MseI T. exiguum
5. PCR product cut by EcoRI 6
PCR product not cut by EcoRI T. pretiosum
6. PCR product cut by MseI ca. 441 bp T. dendrolimi
PCR product cut by MseI ca. 411 bp T. brassicae
7. PCR product not cut by MseI 8
PCR product cut by MseI 9
8. PCR product cut by MaeI ca. 267 and 218 bp T. fuentesi
PCR product cut by MaeI ca. 182 and 113 bp T. galloi
9. PCR product cut by EcoRI ca. 346 and 228 bp T. cacoeciae
PCR product not cut by EcoRI T. lasallei
10 Size of the PCR product < 670 bp 11
Size of the PCR product > 670 bp 14
11. PCR product not cut by MseI 12
PCR product cut by MseI 13
12. PCR product cut by MaeI ca. 480 and 143 bp T. lopezandinensis
PCR product cut by MaeI ca. 517 bp T. iracildae
13. PCR product cut by MseI ca. 390 and 194bp T. rojasi
PCR product cut by MseI ca. 463 and 187bp T. bruni
14. Size of the PCR product > 700 bp T. nerudai
Size of the PCR product < 700 bp 15
15. PCR product cut by EcoRI T. pintoi
PCR product not cut by EcoRI 16
16. PCR product cut by MseI ca. 478 and 196 bp T. acacioi
PCR product cut by MseI ca. 345, 198 and 137 bp T. atopovirilia

Here, ITS2 product size of the seventeen studied species ranged from 379 to 632 bp and sequences differ from each other consistently. In our sample the size of the ITS2 product alone could not identify the species, but cutting the ITS2 product of the different species using only three restriction enzymes (EcoRI, MseI and MaeI) enabled us to identify all species (Table 3). For instance the variation in the size of ITS2 was evaluated in T. atopovirilia, T. exiguum and T. pretiosum. The length of the complete sequences found for T. pretiosum lines all ranged from 410 to 415 bp (Table 3), for T. atopovirilia (561-565 bp) and for T. exiguum (381-383 bp). In general, variation was limited to the number of microsatellite repeat stretches found in the ITS2 sequences.

4 Discussion

Distributed worldwide, Trichogramma are known with circa of 210 species (Pinto, 2006). In North America, Central America and South America are recorded respectively 60, 21 ad 41 species. Considering only the presence of native species in South America, Brazil has the largest number of known species (26) followed by Venezuela (13), Colombia (9), Peru (7), Uruquay (6), Chile (4) and Ecuador and Paraguay (2) (Zucchi et al., 2010; Querino and Zucchi, 2011).

In T. atopovirilia the presence of Wolbachia was recorded for the first time in Brazil by Ciociola Junior et al. (2001a). In all T. pretiosum lines studied here Wolbachia infection was reported for the first time. In South America, Wolbachia infection has been reported in T. pretiosum from Uruguay (Grenier et al., 1998) and from Brazil (Almeida et al., 2001).

The usefulness of endonucleases analysis for distinguishing Trichogramma species was shown by Stouthamer et al. (1999), Silva et al. (1999) and Pinto et al. (2002). How reliable are such molecular keys that are based in some cases on the sequence of only a few lines? The experience with other Trichogramma species is that the intraspecific variation in the ITS2 is rather limited. The variation in number of microsatellite repeats seems to be common in ITS sequences, similar microsatellite variation was for instance also found in Eriophyid mites (Fenton et al., 1997). The range obtained here confirms the low intraspecific variation in the ITS2 length detected by Stouthamer et al. (1999) and Silva et al. (1999). According to Stouthamer et al. (1999) ITS2 can be used for species identification in Trichogramma because the sequence variation within species is small relative to the difference found between species. All morphologically distinct cryptic species are also distinguished by sequence differences, however in the morphologically indistinguishable species North American species T. minutum Riley and T. platneri Nagarkatti no consistent differences were found in their ITS2 sequences (Stouthamer et al., 2000). In this case the ITS2 did not improve upon the morphologically based system. A potential weakness of an ITS2 based molecular key as presented here is that in some cases the key relies on either a restriction of an ITS2 by a particular enzyme or the lack thereof. When no restriction digestion is found it is not immediately clear if the endonuclease worked or if the products lack the restriction site (Stouthamer et al., 1999). It is important to run along a positive control to assure that the restriction reactions work. Molecular laboratories with basic infrastructure can perform PCR amplifications followed by restriction digestions and DNA can be extracted from living, frozen, ethanol-preserved or dried material (Post et al., 1993; van Kan et al., 1996; Ciociola Junior et al., 2000). Just as in case of morphological keys, molecular keys will only be completely reliable once all species of a region are known.

This study together with previous studies (Stouthamer et al., 1999; Silva et al., 1999; Ciociola Junior et al., 2001b) shows the utility of ITS2 sequences in Trichogramma identification. The molecular technique used in this study will be of a great utility in a near future for sequencing species already classified but not sequenced yet from South America. We also expect that with this technique new species will be soon discovered due to the high diversity of Trichogramma host insects in this region. The small number of species found until now is caused by the very limited activities to collect species of Trichogramma. In addition, most of the host species collected were restricted to insects of economic importance. Also there are only a few taxonomists specialized in identifying Trichogramma species using morphological features. Identification by the combination of morphological characters and molecular technique will be of extreme importance for a successful systematic analysis of this group.

Acknowledgements

The authors are thankful to Joop van Lenteren for his valuable comments on the manuscript, and to all those who sent Trichogramma species: Americo I. Ciociola Junior (EPAMIG-Brazil), Aristóbolo Lopez-Avila (CORPOICA-Colombia), Celso L. Hohmann (IAPAR-Brazil), Eduardo N. Botto (IMYZA (CNIA), INTA-Argentina), Mary Whu (SESANA-Peru), Ranyse B.Q. da Silva (ESAlQ-USP-Brazil) and Gary A. Platner (Universisty of California-Riverside-USA). This molecular biology research was financially supported by Embrapa (PRODETAB).

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Received: March 19, 2014; Accepted: June 11, 2014

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