Maruyama et al. BMC Genomics 2010, 11:363 http://www.biomedcentral.com/1471-2164/11/363 engineering tissues [28] and tick silk-like proteins may be more adequate for this purpose because of the intimate relation of this parasite with its host's skin. There are other precedents in nature for our finding that the distribution of distinct GRPs correlates with the biology of metastriate ticks. Spiders, which are also Arachnidae, offer a well known example: the architecture and mechanical properties of different spider webs are correlated with the biological characteristics of their spinners, for example, aerial versus terrestrial capture habits. These properties ultimately rely on the specialized functions of different types of silks. Of interest to studies on the evolution of ticks, orb weaving by spiders is mono- phyletic, having evolved only once and speciation of spi- ders relates to use of different silks [15]. Genes encoding flagelliform silks were thought to be expressed exclusively by modern orb weaver spiders that make more elastic, gluey webs. However it was recently shown that cribellate orb weavers, which make drier webs, also express flagelli- form silk genes [29], albeit in lower quantities. Blackledge and colleagues [15] suggested that an increase in the expression of flagelliform silk genes may have resulted in development of modern orb weavers [15]. Another exam- ple refers to the silks produced by salivary glands of simu- liid filter-feeding flies. Simulium noelleri and S. ornatum use silk pads to attach to substrates, the composition of which varies according the requirements of their habitats: S. noelleri feeds in lake outlets where weaker currents are found and S. ornatum feeds in open waters with stronger currents. Accordingly, there are differences between age- ing processes and biochemical composition of the silk pads from these two species, S. ornatum presenting the most durable structure [30]. A third and final example is offered by larvae of two species of caddisflies. Hydrop- syche angustipennis spins hiding tubes and catching nets that collect food in water currents; larvae of Limnephilus decipiens use silk fiber only for stitching fragments of grass into hiding and pupation cases. The composition of the silk fibers from these species differed by the arrange- ment of motifs in higher order repeats and by the pres- ence of species-specific motifs. Although the amounts of glycine are similar, the H-fibroin of H. angustipennis presents proline containing motifs, whereas L. decipiens presents a highly charged motif, EEGRRR [31]. Conclusions In the present work the differences observed for distribu- tion of glycine-rich proteins were related to the number of hosts visited (i.e., if the species is monoxenous or het- eroxenous) and to the anatomy of mouthparts (long or short hypostome) of three species of metastriate ticks. All ixodid ticks, with the exception of some Prostriate, pres- Page 14 of 17 ent a strategy for attachment, but it differs among them. The species from the genus Amblyomma, which belongs to the Longirostrata ticks, secrete a casing around their long, fully inserted hypostome. In ticks from the Breviro- strata group, which includes species from the genus Rhi- picephalus, the mouthparts are short and barely penetrate in epidermis [3], so a larger cement cone, from which GRPs have been purified [4], is necessary and is deposited in the upper layers of their host's skin. Thus, it seems that the two Brevirostrata ticks, R. microplus and R. sanguineus, need to express more glycine-rich proteins than the Longirostrata tick, A. cajennense, in order to compensate for the small size of mouthparts and for the superficial fixation at the site of attachment. Further- more, R. microplus is monoxenous and R. sanguineus is heteroxenous and comparisons made between these ticks show that the former presents the greatest diversity of glycine-rich proteins, possibly because it is a one-host tick that feeds uninterrupted for many days until comple- tion of its life cycle and, therefore, has greater demands for sustaining its attachment on host skin. Contigs of salivary glands for several other species of ticks have also been examined. While the relative abun- dance of transcripts coding for glycine-rich proteins can- not be accurately compared between salivary gland libraries constructed in different laboratories and under- going different biological situations (for example, infec- tion and feeding time, number of salivary glands used or if whole body ticks were used, etc), it is still noteworthy that annotation of the transcriptomes of salivary glands from female I. scapularis and I. pacificus indicate that prostriate ticks do not rely on glycine-rich proteins as heavily as metastriate ticks for their attachment to hosts or for other biological functions [32,12]. On the other hand, salivary glands of females of D. andersoni, a metas- triate, heteroxenous, Brevirostrata tick, also contain abundant transcripts for GRPs: of the 30 contigs contain- ing the most abundantly expressed ESTs in salivary glands of females of D. andersoni, 9 presented similarities to glycine-rich proteins and contained from 21 to 5 ESTs each [33]. In conclusion, our findings furnish preliminary evi- dence to support the hypothesis that species of ticks with differences in the anatomy of their mouthparts and in the number of hosts they infest during their biological cycle rely on different types and quantities of glycine-rich pro- teins. This hypothesis must be further tested by expand- ing these observations to a larger number of species, by experimental approaches such as RNA interference of expression of selected GRPs and by characterization of isolated GRPs. The data suggests that prostriate ticks rely on their elongated barbed hypostome mouthparts and