Bt Research 2015, Vol.6, No.4, 1-12
4
endogenous plasmids encoders thuringiensins I were
acquired by horizontal gene transfer, in a co-evolution
with insects through a host-parasite relationship.
Representing a unique genetic resource and as part of
adaptive genetic pool can play an important role in the
biology and evolution of
Bt
where cells are hosted
(Schnepf et al., 1998).
(Levinson et al., 1990) also identified another type of
thuringiensin, the type II, in a strain of
Bt
subsp
.
morrisoni
. The thuringiensin of type II is analogous to
uracil (UTP) and presents a greater toxicity than type I,
mainly to Coleoptera (Levinson et al., 1990; Palma et
al., 2014; Perchat et al., 2005). According Saukas et
al., (2014), strains producing thuringiensins type II do
not harbor the gene
thuE
in their plasmids and,
therefore, these exotoxins are encoded by different
genes. However, due to the low number of studies
about this type of toxin, there structure remains
unknown (Rodríguez et al., 2003; Obeidat et al., 2012).
3 Production of tThuringiensin
(Espinasse et al., 2002a) reported that the production of
thuringiensins may occur in response to particular
environmental conditions. Other authors believe that
this secondary metabolite can act as a molecular signal
inside of the microorganism responsible for transcriptional
control of the genes of sporulation (Johnson et al.,
1975). As previously mentioned, the production of
thuringiensins is linked with large plasmids (Levinson
et al., 1990). According (Obeidat et al., 2012), the
plasmidial DNA profiles of
Bt
strains that produce
these exotoxins share three great bands (> 10 kpb), i.e.
the regulatory genes of the synthesis of thuringiensin,
or part of its structure, are encoded by common bands.
(Espinasse et al., 2002b) suggested that the genetic
determinants responsible for producing these toxins
usually found in plasmids Cry-dependent, are regulatory
elements, indicating that any strain of
Bt
, of any
serotype, is able to acquire a plasmid that encodes
thuringiensins by conjugation processes (Obeidat et
al., 2012). Thus, several authors claim that thuringiensin
production is a specific property of strains, rather than
being a specific property limited to certain serovars of
Bt
(Rodríguez et al. 2001; Hernández et al. 2003; Lee et
al. 2001; Ohba et al., 1981; Tanada and Kaya 1992).
Despite the serotyping of
Bt
strains be insufficient to
determine the production of thuringiensins, there is a
strong correlation between production or lack of
production of this exotoxin with serotype (Barfod,
2010). In other words, strains belonging to
darmstadiensis
,
thuringiensis
or
tolworthi
serotypes, which are considered
common producers, will have higher chances of
producing this metabolite than those belonging to
serovars
alesti
,
sotto
,
aizawai
, and
morrisoni
, classified as
rare producers (Rodríguez et al., 2003). In the literature,
it has been reported the production of thuringiensins
in at least 13
Bt
serotypes (Table 1).
4 Toxicity of Thuringiensins
Due to its broad spectrum of nonspecific toxicity to
invertebrates of the orders Lepidoptera, Coleoptera,
Diptera and species of nematodes, which are
representatives of agricultural pests and vectors of the
world's most important diseases (Table 2), thuringiensins
have promising applications in biological control
practices. In addition to these taxa, these exotoxins
also have harmful effects against insects of the orders
Hymenoptera, Isoptera, Orthoptera, Hemiptera and
Neuroptera, plus species of aphids and mites (He et al.,
2011; Liu and Tzeng 1998; Liu et al., 2010; Pinto et
al., 2010; Royalty et al., 1991; Tanigoshi et al., 1990;
Zhou et al., 2013). However, this toxin also has
cytolytic and hemolytic activity against non-target
organisms and vertebrates, acting including in human
erythrocytes (Belder e Elderson 2013; Iatsenko et al.,
2014; Liu et al., 2010; Obeidat et al., 2012; Palma et
al., 2014; Veloorvalappil et al., 2013).
When administered orally, thuringiensins are highly
toxic to a wide range of insect species (Perchat et al.,
2005). In sublethal doses, the effects can be teratogenic
or are involved in critical stages of metamorphosis, as
in the formation of ecdysis, in the transformation into
pupae and in the emergence of adults (Burgerjon et al.,
1969; Espinasse et al., 2002a; Glare and O’Callaghan
2000; Tsuchiya et al., 2002). In some cases, the
duration of the larval stage may be prolonged, or the
lifespan and fertility of the insects can be affected
(Carlberg, 1973). Thuringiensins also prevents the
regeneration of midgut cells of intoxicated insects larvae
(Federici et al., 2010). Among the insects more
susceptible to thuringiensins, are those who belong to
the order Diptera (Carlberg, 1986) and have already
been reported including mutagenic effects in fruit fly
(
Drosophila melanogaster
) (Carlberg 1973).
The mode of action of thuringiensins is quite different
from the mode of action of δ-endotoxin (Vachon et al.,
