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Revista Brasileira de Entomologia

Print version ISSN 0085-5626

Rev. Bras. entomol. vol.57 no.2 São Paulo Apr./June 2013  Epub May 31, 2013

http://dx.doi.org/10.1590/S0085-56262013005000009 

Expression profile of a Laccase2 encoding gene during the metamorphic molt in Apis mellifera (Hymenoptera,Apidae)

 

 

Moysés Elias-Neto1; Michelle P.M. Soares; Márcia M. G. Bitondi

Departamento de Biologia; Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto; Universidade de São Paulo; Av. Bandeirantes 3900, 14040– 901; Ribeirão Preto, SP, Brazil. mmgbit@usp.br

 

 


ABSTRACT

Expression profile of a Laccase2 encoding gene during the metamorphic molt in Apis mellifera (Hymenoptera, Apidae). Metamorphosis in holometabolous insects occurs through two subsequent molting cycles: pupation (metamorphic molt) and adult differentiation (imaginal molt). The imaginal molt in Apis mellifera L. was recently investigated in both histological and physiological-molecular approaches. Although the metamorphic molt in this model bee is extremely important to development, it is not well-known yet. In the current study we used this stage as an ontogenetic scenario to investigate the transcriptional profile of the gene Amlac2, which encodes a laccase with an essential role in cuticle differentiation. Amlac2 expression in epidermis was contrasted with the hemolymph titer of ecdysteroid hormones and with the most evident morphological events occurring during cuticle renewal. RT-PCR semiquantitative analyses using integument samples revealed increased levels of Amlac2 transcripts right after apolysis and during the subsequent pharate period, and declining levels near pupal ecdysis. Compared with the expression of a cuticle protein gene, AmelCPR14, these results highlighted the importance of the ecdysteroid-induced apolysis as an ontogenetic marker of gene reactivation in epidermis for cuticle renewal. The obtained results strengthen the comprehension of metamorphosis in Apis mellifera. In addition, we reviewed the literature about the development of A. mellifera, and emphasize the importance of revising the terminology used to describe honey bee molting cycles.

Keywords: apolysis; cuticle; ecdysteroids; honey bee; pupation.


 

 

Metamorphosis consists in a drastic reorganization of development in which an immature larva becomes a reproductively active adult (Gilbert et al. 1996). The metamorphic process in insects occurs in the context of growth cycles characterized by molts, i.e., the periodic substitution of the old cuticle by a new one, newly-synthetized. After a series of larval molts, which number is variable among species, the larval-imaginal transition in holometabolous insects occurs through two molting cycles: one leading to pupation (metamorphic molt) and the other ultimately resulting in differentiation of the adult (imaginal molt).

The integument of insects consists of an external cuticle, or exoskeleton, that overlies the epidermis (Hepburn 1985). Each molting cycle begins with apolysis, the freeing (detachment) of the epidermal cells from the old exoskeleton (Jenkin & Hinton 1966) and finishes with ecdysis, the eventual shedding of non-profitable portion of cuticle. The morphogenetic period between apolysis and ecdysis is designated pharate (cloaked), which is the phase of an instar enclosed within the cuticle of the previous instar. Exoskeleton differentiation occurs essentially during the pharate period (Hinton 1946).

The sequence of molting events, or molting dynamics, is coordinated by the titer of ecdysteroid hormones (Riddiford 1985; Nijhout 1994) and includes a complex process of cuticle hardening, or sclerotization, in which laccase enzymes have an essential role. Laccases [(EC 1.10.3.2), p-diphenol: O2 oxidoreductases] catalyze the oxidative conjugation of catechols with cuticular proteins (Kramer et al. 2001; Suderman et al. 2006), necessary for cuticle sclerotization. Laccases are supposed to be universally distributed in most various life domains (Claus 2004). They have aroused interest by their industrial applications (Durán et al. 2002) and, in case of their studies in insects, by their biotechnological potential in agricultural pests and disease vectors control (Arakane et al. 2005).

An increasing attention has been paid to insect laccases. Its enzymatic activity has been characterized in the integument of Diptera (Barrett & Andersen 1981; Barrett 1987a,b; Binnington & Barrett 1988; Sugumaran et al. 1992) and Lepidoptera (Dittmer et al. 2009). In addition, the expression of genes encoding laccases has also been described in Diptera (Gorman et al. 2008), Lepidoptera (Dittmer et al. 2004; Yatsu & Asano 2009), Coleoptera (Arakane et al. 2005; Niu et al. 2008), and more recently, in Hymenoptera (Elias-Neto et al. 2010) and Hemiptera (Futahashi et al. 2010, 2011).

If focused on the context of the periodic molts, studies on enzymes and proteins involved in cuticle formation should contribute to a better understanding of metamorphosis at the molecular level. With this goal, we characterized the activity and biochemical properties of a phenoloxidase involved in cuticle pigmentation (Zufelato et al. 2004), and the gene encoding this enzyme (Lourenço et al. 2005). In addition, we described the structure and expression of the genes encoding Laccase2 (Elias-Neto et al. 2010) and a structural cuticle protein, AmelCPR14 (Soares et al. 2007). In these studies, mostly centered on the pupal-to-adult (imaginal) molt, gene expression was approached in the context of the ecdysteroid-regulated molting events.

The purpose of the current study was to describe the temporal expression pattern of the Amlac2 gene during the larval-to-pupal (metamorphic) molt of Apis mellifera Linnaeus, 1758 at the light of the ecdysteroid titer modulation during this stage (Rachinsky et al. 1990). In an attempt to highlight the general expression pattern of cuticle genes during molting cycles, we contrasted the expression of Amlac2 and AmelCPR14 (Soares et al. 2007) during the metamorphic and imaginal molts. This analysis was carried out in the context of the most evident ecdysteroid-regulated molting events, i.e., apolysis, cuticle renewal in the pharate period, and ecdysis.

Finally, we reviewed the literature about the development of A. mellifera, and based on this review, we proposed a reconsideration of the use of the apolysis process as an ontogenetic mark and the replacement of the term "pre-pupa" by "pharate pupa".

 

MATERIAL AND METHODS

Honey bees. 5th instar larvae and pharate pupae of Africanized A. mellifera workers were obtained from hives maintained at the Experimental Apiary of the Universidade de São Paulo, Ribeirão Preto, SP, Brazil. Developing bees were identified as feeding larvae (L5F), spinning larvae (L5S), when they stop feeding in preparation for the metamorphic molt, and pharate pupae (PP). Subphases within L5F, L5S and PP were identified according to criteria established by Michelette & Soares (1993).

Developmental profile of Amlac2. Pools of larvae and pharate pupae were separately homogenized in TRIzol reagent (Invitrogen) for total RNA extraction, according to the protocol recommended by the manufacturer. The extracted RNA was incubated in the presence of RNase-freeDNAse I (Promega) for 30 min at 37ºC to eliminate contaminating DNA. The first strand cDNA was synthetized from a standard amount of total RNA (6 µg), using the synthesis system of SuperScript II (Invitrogen). For cDNA amplification by PCR, we used Master Mix (Eppendorf) and specific primers to Amlac2 gene (forward: 5' GGT ACG CAC TTC TGG CAC G 3' and reverse: 5' CGT CAT GAA ACC GGT GTT G 3'), designed from the predicted sequence (GB11321) in the assembled honeybee genome (The Honeybee Genome Sequencing Consortium 2006). The 273 bp cDNA fragment, flanked by the Amlac2 specific primers was amplified using the following conditions: 2 min at 95ºC, 30 cycles (30 s at 94ºC, 45 s at 58ºC, and 50 s at 72ºC), and a final extension of 10 min at 72ºC. This cDNA fragment was sequenced (Elias-Neto et al. 2010) to confirm gene identity. Amlac2 sequence was deposited in GenBank under the accession number FJ470292.

The cDNA loading control was carried out using specific primers for the gene encoding a cytoplasmic actin (Amact) (GenBank accession number AB023025), which is constitutively expressed during development (Lourenço et al. 2008) (forward: 5' TGC CAA CAC TGT CCT TTC TG 3' and reverse: 5' AGA ATT GAC CCA CCA ATC CA 3'). The thermal cycling program used for amplification of Amact cDNA was the same as described above, except for primer annealing temperature (62ºC) and number of cycles (27). The number of cycles of PCRs was tested for both cDNA sequences (Amlac2 and Amact) and defined in order to avoid saturation. The amplification products were analyzed by electrophoresis in 1% agarose gels containing ethidium bromide. EDAS 290 (KODAK) was used for image analysis.

 

RESULTS AND DISCUSSION

Developmental profile of Amlac2 gene expression. Bee ontogenesis involves the general pattern of holometabolous development, though associated to extremely complex social interactions between brood and adults. The larva undergoes four successive molts, marked by intervals of growth due to abundant feeding. In the fifth larval stage, each brood cell is sealed by a wax operculum produced by worker bees. The larva then stops eating, empties its gut and spins a cocoon, in preparation for the metamorphosis (Michelette & Soares 1993). The metamorphic molt starts with apolysis, proceeds through the pharate period and ends at pupal ecdysis.

We assessed by means of semiquantitative RT-PCR the pattern of Amlac2 expression during the metamorphic molt in A. mellifera, which was contrasted to the ecdysteroid titer variation in hemolymph as determined by Rachinsky et al. (1990) (Fig. 1). The results clearly evidenced that at a certain ecdysteroid titer threshold the level of Amlac2 transcripts notably increases, coinciding with apolysis and onset of cuticle renewal. Amlac2 expression remained high during the pharate period when the pupal cuticle is being deposited, but rapidly declined just before the pupal ecdysis.

 

 

A similar Amlac2 transcription pattern was observed at the subsequent imaginal molt cycle (Elias-Neto et al. 2010). Therefore, independently of the type of the molt, whether larval-topupal or pupal-to-adult, the expression of Amlac2 is induced concomitantly with the increase in ecdysteroid titer that triggers apolysis and cuticle renewal. This is consistent with the function of Laccase2 in cuticle sclerotization, a process occurring during the pharate period of each molt episode.

Other insect models showed a similar transcription pattern of the gene encoding Laccase2. In the lepidopteran Manduca sexta, the gene Mslac2 showed the higher expression in pharate pupae in comparison to the feeding/wandering larvae and 0-day pupae (Dittmer et al. 2004). Similarly, in the coleopteran Tribolium castaneum, the highest levels of TcLac2 transcripts were detected in pharate pupae and pharate adults (Arakane et al. 2005). In these studies, however, the expression profiles were not correlated with the respective ecdysteroid titers.

Reviewing metamorphosis terminology of honey bee: a morphogenetic approach. In addition to Amlac2, other genes with roles in cuticle formation and differentiation are induced during apolysis. As an example, the gene encoding a structural cuticle protein, AmelCPR14 (Soares et al. 2007), showed induced expression associated to the increased ecdysteroid titer that triggers larval-to-pupal and pupal-toadult apolyses (Fig. 2).

 

 

By comparing the course of honey bee development with the modulation of expression of genes involved in cuticle renewal, it becomes clear that apolysis is a hallmark of the intense morphological, physiological and genetic changes that allow the passage to the next stage. Remarkably, the importance of apolysis in ontogenesis has been systematically neglected. The same does not happen with ecdysis, which is conveniently referred in literature as the end point of each molting cycle.

Table I compiles the honey bee literature regarding to the use of apolysis and ecdysis events as ontogenetic marks. As exceptions, Snodgrass (1956), Thompson (1978) and more recently Elias-Neto et al. (2009) recognized the importance of both processes. Another point also related to studies on development is the use, by the respective authors, of the term 'prepupa' instead of the more informative 'pharate pupa'. The term prepupa has prevailed over the pharate pupa, with the exception of Thompson (1978) and Elias-Neto et al. (2009).

 

 

The criterion adopted in this work consists in using both apolyses and ecdyses as marks of development, the first delimitating the ontogenetic status and the last the life stage. Besides, the term prepupa is inappropriate and must be replaced by pharate pupa. This developmental phase corresponds to an ontogenetic sub-division of pupa and not to the last instar larva (for a detailed discussion, see Elias-Neto et al. 2009).

Therefore, we consider this morphogenetic approach in agreement with the reality of the insect ontogenesis. We hope that entomologists and developmental biologists that use bees or other model-insects take into account the importance of considering apolysis in their studies.

The current data add new elements related to the metamorphic molting cycle and contribute for a better comprehension of metamorphosis in Apis mellifera. Moreover, the results shown here gather morphologic, physiologic and genetic evidences that support the adoption of the presented criterion.

 

ACKNOWLEDGEMENTS

We thank L.R. Aguiar for his valuable technical assistance in the apiary, and V.L. Figueiredo for her helpful assistance in the laboratory. This research was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP: 10/ 16380–9), which also provided a fellowship (07/08300–2; 12/ 09108–6) to M. Elias-Neto.

 

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Received 1 October 2012; accepted 6 February 2013

 

 

Associate Editor: Maria Cristina Gaglianone
1 Corresponding author. meliasneto@pg.ffclrp.usp.br

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