Это проблема крупных новшеств в эволюции живого. Как возникает нечто совсем новое?
Там есть давняя загадка - крылья насекомых. Были бескрылые существа, были у них конечности. и вот у них появляются крылья - это новые две пары конечностей, со своими суставами, подвижные, способные к полету... помимо ног.
загадка очень давняя, до сих пор не решена. С другой стороны, каждый шаг для ее решения - это серьезное продвижение в вопросе, как же вообще возникает нечто принципиально новое.
В последнее время появились работы Tomoyasu
Очень интересные попытки. Кратко говоря, ищут связанные с крыльями гены - у насекомых и у других групп. У насекомых нашлись достаточно независимые группы тканей с такими генами, на средне- и заднегруди. Сам Tomoyasu склоняется к интересной морфологической версии - двойного происхождения крыльев. давно уже были две версии - тергальная и плевральная, то есть крылья есть исходно выросты спины (но откуда тогда сочленение, сустав?), либо что это боковые выросты. А Tomoyasu говорит, что одновременно из двух источников, эти зачатки слились. Впервые такую версию высказал в 1981 (если не вру) А.П. Расницын. На совсем других основаниях, конечно, без генов.
Что там происходило функционально - это другой уровень догадок (допустим, планировали, допустим, кря). Пока разбираются с морфологией. Вроде бы удается объединить (пока...0 две ветки гипотез - тергальные гены отвечают за то, что у того, что получается, есть большая плоскость, несущий кусок кутикулы. А плевральные гены - они кодируют кусок конечности у ракообразных... - это в то же время часть жаберного аппарата, когда он формируется у членистоногих - эти плевральные гены отвечают за сустав и за жилочки, - подобные жабрам...
Это крайне огрубленное описание, только чтобы хотя бы поверхностно было понятно. И, вполне возможно, вскоре это изменится - пока исследования наполовину состоят из выражения надежд на будущее
Together, these findings allow us to revise the dual origin hypothesis from an evo-devo perspective, namely, insect wings have a dual origin, and the merger of two unrelated tissues (tergal and pleural) was a key step in developing this morphologically novel structure during evolution. In this hypothesis, we support and expand the idea that was initially put forward by Niwa et al. ; that tergal expansion has provided a genetic mechanism responsible for a large and flat wing blade structure, while pleural plates (potentially with exite-like branches) have provided a complex articulation mechanism along with muscle attachment.
A possible genetic mechanism that facilitated the merger of tergal and pleural tissues is largely unknown. The fusion of the proximal coxopodites into the body wall (i.e. the formation of the pleural plates) that occurred early in the hexapod lineage could have been a key step in placing the two distinct vg-dependent tissues in close proximity. Subsequently, the induction of two vg-dependent tissues adjacent to each other may have caused a ‘cross-wiring’ of the two distinct developmental pathways, resulting in one fused vg-dependent tissue, namely a wing. It is worth mentioning that we have detected two distinct vg-positive cell populations in all three thoracic segments during embryogenesis in Tribolium . This observation suggests that the two types of wing serial homologs are initially induced separately even in the winged segments (T2 and T3) of extant insects (i.e. partial recapitulation of phylogeny in ontogeny). An indepth analysis of how the two distinct vg-positive cell populations induced during embryogenesis contribute to the wing imaginal tissues in Tribolium may help us understand how the merger occurred during the evolution of insect wings.
The evo-devo approach is promising, but has its own limitations. For example, evo-devo analyses do not help determine the selective pressures or the Paleozoic environment that molded the wing from apterygote insect tissues. Evo-devo also does not help reveal the mechanical aspect of the evolution of wings that allowed for efficient flight, nor does it have the ability to reveal the actual shape of the ancient and extinct insects as paleontology can. Instead, evo-devo can provide a new angle to the wing origin debate, which, in combination with other approaches, will provide us with a more comprehensive view of insect wing evolution. The wing origin studies can also impact evolutionary biology in general. Morphological innovation is a fundamental process in evolution, yet the molecular mechanism underlying this process remains elusive. Cooption is often implicated in the evolution of morphologically novelstructures . As discussed, we think that the evolution of insect wings might have been facilitated via a distinct mechanism, i.e. ‘cross wiring’ between two developmental systems that relied on a similar set of genes. The application of evo-devo to the study of insect wing origin will broaden the scope of evo-devo by leading to a more comprehensive view of the molecular mechanisms underlying the evolution of morphologically novel structures.
Taken together, genome-wide analyses in Drosophila have revealed several new aspects of Ubx action during haltere development: first, a surprisingly large number of genes (hundreds, if not thousands) might be controlled by Ubx during haltere development, second, Ubx binds to hundreds of loci in the genome during haltere development, but the act of Ubx binding to these loci alone might not be sufficient for Ubx to regulate the expression of nearby genes, third, the sites that Ubx binds in the genome might, in part, be controlled by chromatin accessibility, which is predetermined by a factor other than Ubx, and fourth, additional factors might partner with Ubx to promote haltere development.
The idea that all wings were Hox-free in the ancestral state was based on the fact that Drosophila forewing (which is a membranous flight wing) represents a Hoxfree state. This view was, in a way, already challenged when the highly modified beetle forewing was found to be a Hox-free state . However, the beetle situation was explainable with the Drosophila paradigm by considering that Ubx acts in the opposite way to that in Drosophila; namely, the evolutionary modification of wings in beetles occurred without Hox input, and Ubx evolved to cancel the modifications in the hindwing to maintain the more ancestral flight wing characteristics. Again, the Apis situation now brings another challenge to the traditional view regarding the Hox-independent nature of the ancestral wings. One interesting point to consider is the origin of insect wings. Although the exact tissues that served as the origin of insect wings are still a mystery (reviewed in ), those tissues could have already been under the control of Hox genes. This view could significantly challenge the current Drosophila paradigm, as Hox genes would have been a part of the wing gene network in the ancestral situation and forewings became Hox-free in some lineages.
The four-winged phenotype of the Drosophila Ubx mutant has been one of the most symbolic figures in developmental biology, epitomizing how influential one gene can be to the development of an organism. The same Ubx phenotype has been quite iconic in evolutionary biology as well, since this phenotype exemplifies that, through genetic manipulation, we might be able to strip away evolutionary modifications from extant organisms to understand the molecular changes that have facilitated morphological evolution. Decades have passed since the four-winged phenotype has been reported, and accumulating knowledge obtained from studies on Ubx in Drosophila has provided us with a framework to investigate the role of Ubx in the evolution of insect wings. Until recently, studies in other insects heavily relied on this framework (i.e. Drosophila paradigm), testing the function of a handful of genes learned from Drosophila studies. However, with the recent advances of molecular biology techniques, we are finally reaching the stage where Drosophila type genetics and genomics are possible even in other non-Drosophila species. By analyzing the function of Ubx in various insects at a level that is currently only achievable in Drosophila, we will be able to obtain a less biased view of the function of Ubx, which will in turn lead to a more comprehensive understanding of the molecular mechanisms that have facilitated the diversification of insect wings.
The lack of wing serial homologs in dipteran insects At least in Drosophila, the situation of wings and their derivatives (dorsal appendages) somewhat parallels the situation of the ventral appendages. As mentioned, morphologically, wings are unique to the two thoracic segments (T2 and T3) in extant insects, including Drosophila. Among the several wing genes identified from Drosophila studies, vestigial ( vg) is often considered one of the most critical wing marker genes because of its wing-specific function during the development of epidermal structures (although vg does have additional functions in tissues outside of wings and halteres, such as muscle 24– 26) and its ability to induce ectopic wings when overexpressed 27– 30. In Drosophila, dorsal appendage primordia (wing and haltere imaginal discs) are induced during embryogenesis. These imaginal discs in T2 and T3, along with a pair of residual cell clusters in T1, are the vg-positive epidermal tissues in Drosophila 27, 31. In contrast, the segments outside of the thorax do not have vg-positive epidermal tissues that contribute to adult morphology 27. Because vg-expressing imaginal tissues are missing in the non-winged segments, it has been considered that the induction of wing-related structures (that is, wing serial homologs) is suppressed in these segments, similar to the absence of the leg serial homologs in the abdominal segments of Drosophila (Hox action 2).
Two important messages can be obtained from the wing (serial) homolog studies: (i) as mentioned, wing serial homologs are widespread, and (ii) wing serial homologs can have drastically different morphologies from each other. For example, in regard to the wing serial homologs of tergal origin, they can be lateral expansions of dorsal body wall or an elaborated helmet in T1, gin-traps (modified body wall) in the abdomen, and wings in T2 and T3. Given that many of the dorsal wing serial homologs are modified body wall structures, the body wall character state appears to be more plesiomorphic (that is, retaining ancestral morphologies) among the dorsal wing serial homologs (albeit with varying degrees of modification), whereas insect wings may be an apomorphic version of this trait ( Figure 1A). A similar argument can be made for pleural wing (serial) homologs, with the proximal leg segments with branches (exites) as the most plesiomorphic, followed by pleural plates of hexapods as a more derived state, and wings as the most apomorphic version of this trait ( Figure 1B). The dual origin hypothesis proposes that the most apomorphic versions of these two traits actually overlap ( Figure 1). This hypothesis is attractive as it can potentially unify the two competing hypotheses; however, both tergal and pleural hypotheses are also valid at this point. Further investigation into wing serial homologs (outlined below) will help differentiate these hypotheses.
A tissue with the expression of vg and of other wing marker genes could have emerged via co-option. It is a challenge to differentiate these tissues from true wing serial homologs. As discussed above, we believe that Hox analysis will be powerful to exclude de novo vg-dependent tissues. Hox LOF mutations allow for transformations among serially homologous structures, while it is less likely that Hox LOF mutations can cause homeotic transformation between the original and the de novo structures evolved via co-option. For instance, the horn of Onthophagus beetles requires the leg gene network for its formation, but it does not transform into the leg upon Hox LOF mutation 22, 23. Thus, it is critical to assess the ability of a tissue to transform into the wing upon Hox mutation before determining whether the tissue is a wing serial homolog.
As mentioned, previous expression analyses for wing marker genes in non-insect arthropods 44, 45 and in non-winged hexapods 46 have shown that evo-devo analyses can provide critical information for the study of insect wing origin, thus establishing a basis for the expansion of our analysis of insect wing origin beyond the winged insects and even beyond Insecta. An essential next step in this direction is to analyze the function of wing gene homologs and their genetic interaction in a diverse array of arthropod taxa ( Figure 2). For example, the tergum and stylus (a pleural structure) of a non-winged insect (bristletail) have been found to express vg 46. It will be interesting to investigate how much of the gene regulatory network operating in these tissues is shared with insect wings. Functional analyses in various crustaceans will also be beneficial to further identify tissues that share ancestry with the insect wing (that is, wing homologs). Several leg branches (homologous to a pleural lineage) in the brine shrimp and the crayfish have been shown to express some wing marker genes 44, but their functional dependency on wing marker genes, including vg, still needs to be tested. In addition, it is yet to be determined whether other tissues in these crustaceans (such as terga) also share gene regulatory networks with the insect wing. Furthermore, given the vast diversity and the possible polyphyletic nature of the crustacean order 57, 58, it will be critical to analyze more crustacean species. Myriapoda (millipedes and centipedes) is another taxon that may provide interesting insights in regard to identifying tissues homologous to insect wings. The possible wing homologs in Myriapoda have not yet been investigated. However, some myriapods possess elaborated tergal expansions reminiscent of paranotal lobes. Therefore, it would be interesting to investigate whether these structures (and other tissues such as parts of the leg) have dependency on genes homologous to insect wing genes. The myriapod lineages are even more basal on the arthropod phylogeny than crustaceans 58, and thus identification of potential wing homologs in the myriapod lineages can provide crucial information as to which tissues have given rise to insect wings. In summary, functional analyses for genes homologous to wing genes in a diverse array of arthropod taxa will lead us to a better understanding of what tissues are homologous to wings in these lineages, which will help us further evaluate the wing origin hypotheses from an evo-devo perspective.