Role of hunchback in Spider Achaearanea tepidariorum
Wim Damen’s lab in University of Cologne is interested in understanding the evolutionary basis of biological pattern formation using spider’s as model system. The chelicerates to which the spiders belong form a monophyletic group that has already split during the Cambrian from the other arthropod groups. Damen’s group famous for studying Spider’s started with Cupiennius salei,a tropical spider and now shifted to another spider species Achaearanea tepidariorum. These spiders can currently be found all over the world. In these spiders males are smaller than the females and range from 3.8 to 4.7 mm in length.
Evelyn E. Schwager and colleagues recently published their results related to hunchback gene in online edition of current biology ,where they show that hunchback acts as a segmentation gene during anterior patterning of a non insect arthropod, the spider Achaearanea tepidariorum.Previous work from many laboratories have shown beyond doubt that in insects, the gap gene hunchback (hb) is required for the formation of a set of adjacent segments and this function is performed by proper regulation of downstream target genes of the pair rule and segment-polarity classes. In addition, hb is a major regulator of Hox genes and it has been suggested that this is the ancestral role of hb in insects or perhaps even arthropods.
As mentioned before that chelicerates form a monophyletic group and that it had already split during the Cambrian from the other arthropod groups. So comparisons between spiders and insects will help us to gain more insights into the degree of conservation and divergence during arthropod evolution. Any embryological feature that is conserved between spiders and insects is likely to represent an ancestral feature for arthropods.
As in the case of many genes which are vital for proper embryonic development , information pertaining to the function of hunchback was also known from studies performed mainly in insects and their role in non insects was studied for the first time by Damen’s group.
The cells of the blastula are uniform in shape and size but as one move along the ventral region of the blastula cells tend to thicken and enlarge to form “Germ band”.This germ band grows and enlarge in all insects regardless of its beginning size. The initial size of germ band varies in insects and can be divided into three groups :
1) Long germ band insects which include the Diptera (Drosophila/flies), Lepidoptera (butterflies and moths), Hymenoptera (bees, wasps, ants), and some Coleoptera (beetles) are those insects where germ band extends through the entirety of the egg. In Drosophila, a regulatory cascade controls the simultaneous formation of all segments, meaning this long germ band contains the primordia of all the segments that will form in the embryo (and adult).
2) This is in contrast to short and intermediate germband insects where only the most anterior body regions are specified by the blastoderm stage. Posterior segments are specified later in embryogenesis during a period of germband elongation by virtue of posterior proliferation zone that will bud off new cells to form the additional segments. eg : Tribolium.
It was always believed that anterior and posterior segmentation in arthropods might employ different mechanisms and that Drosophila segmentation is derived from such a dual system.
Hunchback in Spider Achaearanea tepidariorum (At-hb) is expressed maternally in developing oocytes and is first detectable in the blastopore region of stage 3/4 embryos. To understand the function of hunchback , authors used parental RNAi to knockdown the expression of At-hb. The progeny showed reduced pairs of legs 2 or 3 ,when compared to normal 4 pairs and this is due to loss of segments. Interestingly, many of these first instars actually survive the next molt and are able to walk ( you can see nice videos of these spiders trying to walk using thier two pairs of legs in the supplementary figs section of the article ).
Furthermore authors show that At-hb is required for the proper organization of target genes in this region of the embryo, suggesting that At-hb acts as a gap gene in the spider. However in contrast to insects, hunchback does not control Hox gene expression in the spider. Based on these findings from RNAi experiments of hb authors conclude that that hb might have had a segmentation gene function in the arthropod ancestor and also contradicts the suggestion that the control of Hox genes is the ancestral role of hunchback. Their results also helped to elucidate use of two different mechanisms in short germ band insects to segment their anterior and posterior body parts. Spiders employ a Drosophila-like genetic mode for anterior segmentation and a vertebrate like mechanism involving Wnt8 and Notch/Delta signaling to pattern posterior segments.
Reference :
hunchback Functions as a Segmentation Gene in the Spider Achaearanea tepidariorum.
Schwager EE, Pechmann M, Feitosa NM, McGregor AP, Damen WG.
Curr Biol. 2009 Jul 22.
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