I N S E C T S 579

[See also Disease, article on Infectious Disease; Im-mune System, articles on Microbial Countermeasures toEvade the Immune System and Structure and Functionof the Vertebrate Immune System; Plagues and Epidem-ics; Vaccination; Viruses.]

B I B L I O G R A P H Y

Bush, R. M., C. A. Bender, K. Subbarao, N. J. Cox, and W. M. Fitch."Predicting the Evolution of Human Influenza A." Science 286(1999): 1921-1925. Use of molecular techniques to study anti-genie drift.

Cox, N. J., and C. A. Bender. "The Molecular Epidemiology of In-fluenza Viruses." Seminars in Virology 6 (1995): 359-370.

Cox, N. J., and K. Subbarao. "Global Epidemiology of Influenza:Past and Present" Annual Review of Medicine 51 (2000): 407-421.

Dowdle, W. R. "Influenza A Virus Recycling Revisited." Bulletin ofthe World Health Organisation 77 (1999): 820-828.

Glezen, W. P., and R. B. Couch. "Influenza Viruses." In Viral Infec-tions of Humans, edited by A. S. Evans and R. A. Kaslow, 4thed., 473-505. New York, 1997. General overview of influenza.

Murphy, B. R., and R. G. Webster. "Orthomyxoviruses." In FieldsVirology, edited by B. N. Fields, D. M. Knipe. and P. M. Howley,3d ed., 1397-1445. Philadelphia, 1996. General overview of in-fluenza.

Nicholson, K. G., R. G. Webster, and A. K. Ray (eds). Textbook ofInfluenza. Oxford, 1998. In-depth chapters covering many areasof influenza biology.

Reid, A. II., J. K. Taubenberger, and T. G. Fanning. "The 1918 Span-ish Influenza: Integrating History and Biology." Microbes andInfection 3 (2001): 81-87.

— ROBIN M. BUSH

INSECTS

Insects are the most diverse group of organisms on earthand seem to have been diverse since at least the Permianperiod, about 250 million years ago. This qualifies themas the most successful animals ever to have lived onearth. Scientists recognize at least 750,000 species of in-sects and place them in the order Insecta. They estimate,however, that the total number of living insect speciesis ten million or more. The number of beetle speciesalone (order Coleoptera) at about 500,000, is roughlytwice that of the nearest other major group (greenplants). This amazing richness is evidence that evolutionhas taken varied paths to fill or subdivide niches. Larger(and smaller) organisms seem not to have exploitedniches to this extent. Many researchers have posed thequestion Why are there so many insect species? Thisquestion may be rephrased as What key adaptationshave allowed insects to be so species rich?

Insects as Arthropods. Insects are the most spe-cies-rich class in the phylum Arthropoda. Arthropodsinclude major groups such as the extinct trilobites, cheli-cerates, crustaceans, myriapods, and insects. The phy-lum Arthropoda contains roughly three-quarters of the

species of animals on earth. Insecta alone accounts forabout two-thirds of the animal species (Hammond,1992). Researchers have diagnosed the phylum Arthro-poda in different ways. The most generally accepted fea-tures are a chitinous exoskeleton and jointed append-ages. Other commonly cited synapomorphies (shared,derived homologous features) include an open circula-tory system, Malpighian tubules used for nitrogenouswaste concentration and excretion, the hemocoel bodycavity, the lack of cilia on any body cells, the lack ofnephridia (the excretion organ of segmented worms),and separate sexes (as opposed to a near sister group,segmented worms, which can be hermaphroditic). Someexperts have included the lobopods, Peripatus, for ex-ample, as arthropods but most recognize those interest-ing creatures as a separate phylum, the Onychophora.The exact topology of relationships among major groupsof the phylum Arthropoda is a contentious issue. Thephylogenetic tree in Figure 1 summarizes the relation-ships among major groups of arthropods. Note that thesister group of insects is myriapods. Other recent mo-lecular considerations list Crustacea as the sister group.Support for this arrangement with Crustacea is sparseand does not overwhelm the purported synapomorphiesof Insecta and Myriapoda when morphological andmolecular lines of evidence are evaluated together(Wheeler et al., 2001). Whether myriapods form a mono-phyletic group (a group consisting of a common ances-tor and all of its descendants) is unclear. Myriapodscould be merely a paraphyletic group (an evolutionarygrade consisting of some, but not all, descendants of acommon ancestor). Both points of view have propo-

FIGURE 1. Relationships Among Classes of Arthropoda.C. Ritey Nelson.

Annelida

Onychophora

Arthropoda

Myriapoda

Insecta

580 I N S E C T S

nente. In any event, the ancestral insect (hexapod) isthought to have evolved from some member of this myr-iapod group. The controversy, in part, relates to exactlywhich myriapod, whether millipede, centipede, symphy-lan, or pauropod, shares a common ancestor with in-sects.

Insects as Hexapods. A well-supported tree show-ing phylogenetic relationships among living six-leggedarthropods is shown in Figure 2. To produce this ar-rangement, Wheeler and colleagues (2001) reviewed thevast morphological literature and added charactersgiven by sequences from two genes. It has become com-mon practice in recent years to list, the class to whichinsects belong as Hexapoda. But a rationale for recog-nition of all six-legged artliropods as class Insecta is sup-portable as well. A primary reason for recognizing classHexapoda (rather than class Insecta) is generally givenas the lack of resolved phylogenetic relationships amongCollembola, Diplura, and Protura. Advocates maintainthat a named basal node will clarify the relationshipswith taxa further up the tree. Also, a need is articulatedto recognize individual names for nodes from which Ar-chaeognatha, Zygentoma, and winged insects emanate.The five groups of wingless animals in question here havebeen included in the "Apterygota" in the past. Apterygote,in this sense, is clearly a paraphyletic group, and its useas a formal clade name should be discouraged. Most(but not all) recent reviews of the classification statusof these five orders list Collembola, Diplura, and Proturaas a monophyletic group, Entognatha This status isbased on the presence of mouthparts that can retractinto a facial pouch, as well as several other character-istics. Archaeognatha and Zygentoma each form mono-phyletic groups (Figure 2). The rationale for use of thetaxon name Hexapoda does not hinge on attempts toavoid paraphyletic groups. The second rationale, nam-ing of every node in a phylogenetic sequence, has beencriticized regularly in the past as redundant and unnec-essary. On a more positive note, several characters sup-porting monophyly of Hexapoda (major body regions ofhead, thorax, and abdomen; six legs) have a long tradi-tional use as defining characteristics for Insecta in thegeneral scientific community and for the public as well.Although "tradition" alone is not a valid argument for aparticular naming scheme, stability of use is desirable.Here, class Insecta is used for the node connecting theEntognatha with the remainder of six-legged arthro-pods.

Insect Phylogeny and Key Innovations. We canbegin to answer questions such as "Why are there somany kinds of insects?" by considering a few key ad-aptations. Six breakthroughs in morphological adapta-tion are largely responsible for the success of insects.(A few others have been important for insects on asmaller, but, significant, species-richness scale). The sixmajor morphological innovations of insects are having

six legs; tagmatization, having the body divided intothree regions; mandibles with two condyles; wings; theability to compactly fold these wings; and complete met-amorphosis.

The oldest insect fossils are pieces of Collembolafrom the Rhvnie Chert of Scotland. Several specimensincluding the described Rhyniella precursor have beenfound in these lower Devonian deposits dated to ap-proximately 400 million years ago. These specimens arethe first in the fossil record to show the six-legged con-dition and having the body segmentation coalesced intothree main body regions: head, thorax, and abdomen.Thus, the minimum age of the insect clade is 400 millionyears.

Numerous authors have speculated on the advantagethat six legs and three body regions might have con-ferred on these species. The dual tripod gait, with six legsallows significant stability with few contact points onthe substrate. Functional morphologists have studiedthis phenomenon, as have mechanical robot designers.A metachronal gait is employed by most terrestrial ar-thropods. It is typified by lifting one leg at a time fromthe substrate while keeping the leg behind it down tobear the animal's weight. This allows good stabilitywhile decreasing net energy use. By decreasing the num-ber of legs to six (from the myriapod condition of many),mechanical simplification is achieved. Although it is in-tuitive that simplification with no loss of stability orspeed could give an organism a selective advantage, wemust note that other gait systems work. Is the six-leggedcondition a breakthrough adaptation regarding diver-sity? Not necessarily, but it could have allowed chan-neling of energy and integration resources elsewhere.Resources needed for multiple limbs could be divertedto other important features or functions necessary forsurvival and reproduction. Additionally, limbs freedfrom use during locomotion could take on new roles asmouthparts for enhanced food acquisition and manipu-lation. Ancestral legs could also be modified into com-plex genitalia and enhanced reproduction.

Tagmatization, the specialization of different regionsof the body for different functions, is not unique to in-sects, arthropods, or protostomes for that matter. Whatis unique for the head, thorax, and abdomen arrange-ment seen in insects, however, is a significant simplifi-cation of the annelidlike or myriapod model. The headin insects is largely responsible for environmental sen-sory perception and food acquisition. The thorax bearslegs and wings in insects. It can be considered the lo-comotion center of the animal. In contrast, the abdo-men s particular functions of food processing and wasteremoval can be considered more as retained ancestralconditions. The complex genitalia composed in largepart of modified legs, however, can clearly confer repro-ductive advantage (Eberhard, 1985).

A second condyle or contact point in the mandibles

INSECTS 581

Insecta:1: Six legs;2: Tagmatization:

Head, thorax& abdomen

Dicondylia3; Dicondylic mandible

.Crustacea• Myriapoda

CollembolaProturaDiplura

• Archaeognatha• Zygentoma

Pterygota:4: Wings I

Neoptera:5: Folding wings

Holometabola6: Complete metamorphosis"

EphemeridaOdonata

OrthopteraPhasmida

.— PlecopteraI—Embiidina

GrylloblattariaDermapteraZorapteraIsopteraMantodeaBlattaria

HemipteraThysanopteraPsocopteraPhthiraptera

ColeopteraNeuropteraMegalopteraRaphidiodeaHymenopteraTrichopteraLepidopteraSiphonapteraMecoptera

I—Streps,pteraI— Di ptera

FIGURE 2. Relationships Among Orders of Insecta, with Key Innovations.C. Riley Nelson.

of Zygentoma and the remainder of insects (Dicondylia)conferred increased mechanical advantage with similarmuscle mass. This was a key adaptation because it al-lowed exploitation of tougher food sources.

Wings and active flight confer habitat location, food-finding, dispersal, and mate-locating advantages to theanimals that have them. Active flight has evolved at leastfour times in animals: insects, pterosaurs, birds, andbats. Each of these groups is species rich when com-pared to their nonflying immediate ancestors. The fossilhistory of wings begins with the insects of the Carbon-iferous period, about 320 million years ago. The wingedclade of insects is by far its most species rich, with at

least 750,000 extant species capable of active flight. In-sects are the only arthropod group employing activeflight, and they are the most species rich as well. Of allthe key adaptations of insects, flight is the feature thatis probably most responsible for diversification and suc-cess.

Where did these wings come from? In vertebrates,clearly the wings are modified forelimbs. Pterosaurs,birds, and bats all fly (or flew) by modifying these limbsinto broad, lift-generating surfaces. This is not true ininsects. The wings of insects cannot easily be associatedwith an/ preexisting features. One of the most interest-ing, and at times acrimonious, debates in insect evolu-

582 I N S E C T S

tion centers on the origin of their wings. A recent sum-mary of wing origin theories is given by Dudley (2000).He also discussed theories related to function at inter-mediate stages of wing development in insects. Two ba-sic theories, each supported by extensive conjectureand limited data, are most commonly discussed in theorigin of the actual wing structure. The paranotal exten-sion hypothesis alleges that the wings originated as thindorsolateral extensions of the individual segments of thethorax. The greatest evidential support for the paranotalhypothesis comes from the existence of extensiveflanges in some large Palaeodictyoptera of the Carbon-iferous period, approximately 300 million years ago. Thelocation of wings and protowings at the appropriatenear-head location with respect to center of mass alsolends credence to this hypothesis. The most apparentproblem with this hypothesis is that the extensionswould bear neither articulation nor musculature neces-sary for napping flight. The second major summary, thepleura! hypothesis, claims that the wings are modifiedportions of leg appendages. In one representation of thishypothesis, the wings are thought to be modified gillsenlarged to take on flight capabilities. Support for thishypothesis is greatest because of the readily apparentblade, articulation, musculature, and venation that ex-ists in gills of extant Ephemerida and some extinct basalinsect orders. One difficulty with this hypothesis is thatthese types of gills are never on the thorax, where thenecessary moment to center of mass would need to befor flight. Additionally, this hypothesis would requirewings to have originated in aquatic insects, despite thebest evidence pointing to the wingless ancestor beingterrestrial. Another permutation of the pleural theory,championed by Kukulova-Peck (1983), notes the exis-tence of exite lobes on the basal parts of legs of someinsects of the Carboniferous period. She suggested thatthese exites have broken away from the main leg sup-port structure and migrated dorsally in the body wall ofthe insect. These parts, with associated articulation andmusculature, went on to become wings. Detractors ofthis hypothesis cite the lack of direct evidence of actualstructures in the fossils. They also note that the im-mense physiological and ontogenetic changes necessaryto accomplish such a radical migration of tissue makesthe exite hypothesis virtually untenable.

Besides speculations about the physical origin ofwings, many have considered the functional history ofwing development. The crux of the issue centers on howshort protowings of limited flight enhancement capabil-ity could be modified by natural selection to produce themagnificent flight organs we see today. Again, two basichypotheses are evident. The first is that protowings andwing have always (or nearly always) been under directselection for aerodynamic function. Second, protowingsinitially had other more important functions. Later theywere adapted for aerodynamic function.

The aerodynamic hypothesis maintains that gliding orflight was the selective issue throughout the wing elon-gation process. That even short wings confer benefitfrom an aerodynamic perspective on the way to pow-ered flight has been substantiated experimentally: itcould have happened. A related variation of this hypoth-esis purports that ancestral preflying insects could haveused protowings as sails to skim across water surfaces.

The second set of hypotheses, grouped together aspreadaptation hypotheses, insists that a purpose otlierthan flight and locomotion initiated wing developmentand elongation. Flight, according to adherents, only be-came important in wing evolution at later stages. Variousideas for the initial value of protowings have been pro-posed: thermoregulation, courtship display, warning dis-play, jump escape benefits, and apparent size increasefor predator deterrence.

Note that many of these hypotheses are not neces-sarily mutually exclusive. This may explain the lack ofconsensus on the issue of wing origin and function. Thediversity in form and function that we see in currentinsect wings may arise from a diversity of function his-torically. In any case, wings are obviously important foraerodynamic reasons (movement, dispersal, mate find-ing, food acquisition, and predatory escape) as well asfor a variety of other purposes.

Despite the advantages of wings and flight, they comeat a cost. The wing surface necessary to generate liftmust be large relative to the organism's total size a^dweight. The beating surface can be made lighter throughthinning but must remain expansive to generate enoughlift. These expansive beating surfaces then can becomea burden when predators attempting to evade, evenwhile simply maneuvering as the insect feeds or mates.One solution to this problem would be to fold the wingsneatly out of the way when not in use. This is what hasoccurred. Another solution would be to dispense withwings. Tlus has also occurred repeatedly. Thus, we notethat a variety of insects, scattered all over the phyloge-netic tree, have become flightless.

The final key innovation to be discussed is that ofholomotaboly or complete metamorphosis. The Holo-metabola, holometabolous insects, contains about 85percent of the species in the group. If diversification isa measure of success, then holometaboly could be thesingle most important factor because it is used by somany species. The postembryonic developmental re-gime that holometaboly follows consists of the passagefrom egg through several stages of larvae, then a quies-cent pupa followed by the adult. Such a complex patternis energetically, morphologically, and ecologically un-parsimonious. It comes at great cost. Why, then, wouldsuch a system evolve? What are the benefits of hole-mctaboly? The key function that holometaboly accom-plishes is segregation of immatures from adults. This isaccomplished by insertion of the pupal stage between

I N S E C T S 583

other immature stages and the adult. This juvenile/adultsegregation correlates with several ecological incentives:decreased intraspecific competition, predator avoidance,separation of feeding and reproducing stages, and allow-ance for massive reorganization from juvenile to adult.

Intraspecific competition can be the most severe typeof competition. Complete metamorphosis may reduce itby segregating the juvenile and adult resources, such asfood and shelter. This decrease in competition hypoth-esis is generally cited as the most important reason forthe success of insects having holometabolous develop-ment. However, many hemimetabolous insects also seg-regate these stages in an analogous way that can reduceintraspecific competition. Most prominent in this regardare those that have aquatic immatures and aerial adults.This simply indicates that several pathways to segrega-tion have been used with success. A second professedreason for segregation of immatures and adults is that,by selecting different forms for the two stages, predatorscannot key in on a uniform search image. One or theother stages may avoid detection and escape. The thirdconcept of functional segregation of feeding and repro-duction into two life stages allows for specialization ofmorphological features more advantageous in eachstage. This requires, then, a radical reorganization ofform, the fourth idea given above. Again, these hypoth-eses are not mutually exclusive but may all contributeto the success of the organisms.

Other Innovations and Relationships. Whetherthe first insects lived in water or air is a point of con-tention among evolutionary biologists. This controversyand the available evidence were summarized by Pritch-ard and colleagues (1993). They considered evidence asdiverse as osmoregulation, locomotion, fossils, phylog-eny, gills, life history, and the morphological arrange-ment of the trachea! system. The researchers concludedthat a terrestrial origin of hexapods, and thus insects, ismore reasonable.

The rise to dominance of flowering plants is regularlysuggested as causing the diversification of the insectlineage. Many insects are closely tied to plants by mor-phologies related to feeding guilds such as defoliators,phloem suckers, and pollinators. In considering the di-versity of life on earth, two forms dominate: green plantsand insects. A coevolutionary system in which plantsand insects both diversify in response to the other mustthus be seriously considered. Ehrlich and Raven (1964)proposed a coevolutionary "arms race" between butter-flies and plants. They used this idea to explain the di-versity of secondary compounds found in plants and theclose relationships many insects have with plants. Thefamily-level diversity of fossil insects, however, was al-ready growing exponentially before the angiosperms ap-peared in the fossil record (Labandeira and Sepkoski,1993). In fact, the rate of family-level diversification de-creased when angiosperms came on the serene. Taken

together, this information from the fossil record actuallyshows that diversity of insects and angiosperms is de-coupled. Other analyses, however, have shown thatmany herbivorous lineages of insects have diversifiedwhen compared to their nonherbivorous sister groups.The support for a close insect/plant coevolutionarytrend in diversification therefore is equivocal. At a min-imum it has not had as direct an influence on the diver-sification of insects as is often proposed.

Social insects, including ants and termites, are dom-inant biological forces on earth, especially in tropicalregions. The numbers, biomass, and density of tropicalants and termites are nothing less than astounding. Forexample, ant and termites compose one-third of the ani-mal biomass in an Amazonian rain forest, and by addingtwo other social groups of insects this number reaches75 percent. Ants alone in these systems are diverse, withup to 128 species in 250 square meters. However, on aworldwide scale, considering the amazing diversity ofall insects, ants and termites are not outrageously spe-cies rich. They consist of about 11,500 species world-wide. Still we must note that this is roughly equivalentto the 9,000 species of known birds. In any event, thesesocial insects are extremely successful. Whether theevolution of societies is directly responsible for that suc-cess is uncertain.

Insects are not important components in marine eco-systems. This is enigmatic when one considers how im-portant they are hi all other major ecosystems. Manyinsect groups have evolved the morphological and phys-iological adaptations needed to exploit these habitats,but it is thought that the exquisite range of adaptationsto the terrestrial habitat has hindered them in attemptsto colonize the seas. Crustaceans, trilobites, and otherarthropods radiated in Devonian and Carboniferousseas. This gave them a preemptive competitive advan-tage over the insects, which continues to this day. So thereasons why there are no marine insects may be thatthey did not reach the seas initially and were burdenedwith terrestrial adaptations when returning later.

Conservation. The terminal Permian extinction (240million years ago) devastated insect diversity. Of thetwenty orders of insects in the Permian era, only thir-teen survived into the Triassic period. Equally devastat-ing losses at family and generic levels were sustained.Diversity worldwide in most other animal groups par-alleled this most prominent extinction event the worldhas ever known. Insects passed unscathed through allother extinction events, such as the terminal Cretaceousevent that eliminated large dinosaurs. But insects todaymay be facing the greatest extinction force of their 400million-year history. The indications are that insect spe-cies are disappearing from our planet in numbers thatovershadow those of the end-Permian event. Causescited for their disappearance are habitat destruction andindiscriminate use of pesticides. Insects have a long his-

584 I N T R O N S

tory of evolving key innovations to deal with novel chal-lenges. Already we see evolution occurring in the formof pesticide resistance over a very few years. Is thisenough? Will insects survive these new human-inducedchallenges? Some species, but probably not most, willsurvive. Are we willing to lose the incredible beauty inform, function, and strategy that insects in their hyper-diversity present to us? Are we willing to replace thisdiversity with resilient cockroaches, house flies, and fireants?

[See also Animals.]

B I B L I O G R A P H Y

Dudley, R. The Bio mechanics of Insect Wight: Form, Function,Evolution, Princeton, 2000. An up-to-date summary of the in-sect wing origin and early function controversy.

Eberhard, W. G. Sexual Selection and Animal Genilaiia. Cam-bridge, Mass., 1985. An introduction to the the incredible di-versity of animal genitalia with an interpretation that it is theproduct of sexual selection.

Ehrlich, P. R., and P. H. Raven. "Butterflies and Plants: A Study inCoevolution." Evolution 18 (1964): 586-608. The early intro-duction of coevolution and arms race ideas.

Hammond, P. M. "Species Inventory." In Global Biodiversity,Statu.v of the Earth's Living Resources, edited by B. Groom-bridge, pp. 17-39. London, 1992. A compilation and enumera-tion of living species.

Holldobler. R., and E. O. Wilson. The Ants. Cambridge, Mass., 1990.Contains an introduction to the diversity, abundance, and eco-logical importance of ants, termites, and other social insects.

Kukulova-Peck, J. "Origin of the Insect Wing and Articulation fromthe Arthropodan Leg." Canadian Journal of Zoology 61 (1983):1619-1669. An interpretation of structures from the side (pleu-

~ron) of Carboniferous fossils.Labandeira, C. C., and J. J. Sepkoski. "Insect Diversity in the Fossil

Record." Science 261 (1993): 310-315. A discussion of family-level insect diversity in the fossil record. Clarifies that insectfossils are neither rare nor necessarily poorly preserved.

Pritchard, G., et al. "Did the First Insects Live in Water or in Air?"Biological .Journal of the Linnean Society 49 (1993): 31-44. Aparticularly clear summary of the issue, with a ranking of thevalue of different evidence germane to the aquatic/terrestrialorigin question.

Wheeler, W. C., et al. "The Phylogeny of Extant Hexapod Orders."Cladistics (2001). A summary of morphological and moleculardata used to support a phylogeny for the insects. Contains acomplete bibliography of the subject.

— C. RILEY NELSON

INTERSPECIFIC COEVOLUTION. See Coevolution.

INTRONS

One of the biggest surprises in the history of molecularbiology came in 1977, when it was discovered that genesin eukaryotes are interrupted by noncoding regionsknown as tntrons. Introns are transcribed along with the

coding region during the synthesis of messenger RNAand have to be removed, or spliced, by a protein-RNAcomplex called a spliceosome, before the transcript canbe translated intro a protein (see Vignette). The regionsof the gene that are spliced together to create a func-tional transcript are known as exons. Sequences thatinterrupt genes are also found in bacteria and the ge-nomes of organelles such as mitochondria and chloro-plasts. However, such introns are self-splicing parasiticelements, and it is the evolution of eukaryotic, or spli-ceosomal, introns that is the focus of this article.

The number and size of introns varies enormouslybetween genes and organisms. In yeast, most genes areuninterrupted, whereas mammalian genes typically con-tain multiple introns, which together make up the vastmajority of DNA within a gene. For example, the genecoding for dystrophin in humans, mutations in whichcause Duchenne muscular dystrophy, consists of sev-enty-nine exons spread over about two million basepairs of DNA. Less than 1 percent of the gene, whichtakes up to sixteen hours to transcribe, is present in thespliced transcript. Even more remarkable, the introns ofsome genes are so large that other unrelated genes arelocated within them.

Introns and Gene Regulation. The location of in-trons in coding sequences is associated with certainshort sequence motifs, known as splice sites, which arerecognized by the spliceosomal machinery. Typically, in-trons begin with the nucleotide sequence GU and endwith the pair AG, but there are other patterns, both inthe intron and the adjacent coding regions, associatedwith splice sites (see Vignette). Regulatory sequencesmay also be found in introns. For example, expressionof the lg$2r gene in mice is prevented by the binding ofa represser to a site within the second intron.

One type of gene regulation, called alternative splic-ing, depends critically on the presence of introns. Alter-native splicing is the production of different transcriptsfrom a single gene, by the splicing of alternative sets ofexons. Alternative transcripts may be produced in dif-ferent tissues or at different stages of development.Many transcripts differ only in the first exon, which isusually untranslated but contains regulatory elements.Some genes, however, can produce completely differentproteins through alternative splicing. For example, twoof the key genes involved in the sex-determination path-way of the fruit fly (Drosophila melanogaster), double-sex and transformer, have different forms expressed indeveloping male and female embryos. Another remark-able example of the use of introns occurs during thegeneration of antibody diversity in vertebrates, in whichgene rearrangements within the loci coding for immu-noglobulin genes create a vast diversity of sequencesfrom a library of exons.

However, for most introns, the majority of the nucle-