SIBLING SPECIES,CALL DIFFERENCES,AND SPECIATION IN GREEN LACEWINGS (NEUROPTERA: CHRYSOPIDAE: CHRYSOPERLA)

Green lacewings of the morphologically homogeneous carnea-plorabunda-downesi group within the chrysopid genus Chrysoperla produce unique, species-specific, substrate-borne songs during courtship and mating; both sexes sing, and partners must reciprocally exchange their acoustical signals before copulation will occur. Two widespread, sympatric North American representatives of this complex, the sibling species pair C. plorabunda and C. downesi, hybridize readily in the laboratory but not in nature. This species pair has been presented as exemplifying sympatric speciation by disruptive selection. Here, it is shown from tape-playback and female-choice experiments that calls represent bona fide reproductive isolating mechanisms between the two species. Furthermore, call analyses of F₁, F₂, F₃, and backcross progeny of the two species confirm polygenic control of call expression, in which different alleles at each of several loci are fixed in each taxon. Sex linkage of traits is absent, but the various features of the calls are not completely independent of one another in their patterns of inheritance. These and other life-history data cast doubt on several major premises of the sympatric speciation hypothesis and suggest that call alteration might have triggered the speciation event giving rise to the siblings. A complex of cryptic “song morphs” physically and ecologically identical to C. plorabunda and C. downesi, but singing different songs, exists in the mountains of western North America, while the Alps of central Europe harbor populations of C. carnea that have undergone call differentiation in an analogous but independent manner. It is proposed that call divergence may in itself be driving the speciation process within this section of Chrysoperla, by greatly accelerating the rate at which full reproductive isolation between populations can be achieved.     

Genetics of sexual isolation based on courtship song between two sympatric species: Drosophila ananassae and D. pallidosa

Sexual isolation has been considered one of the primary causes of speciation and its genetic study has the potential to reveal the genetics of speciation. In Drosophila, the importance of courtship songs in sexual isolation between closely related species has been well investigated, but studies analysing the genetic basis of the difference in the courtship songs associated with sexual isolation are less well documented. Drosophila ananassae and Drosophila pallidosa are useful for studies of sexual isolation, because of their sympatric distribution and absence of postmating isolation. Courtship songs are known to play a crucial role in sexual isolation between these two species, and the female discrimination behaviour against the courting male has been revealed to be controlled by a very narrow region on the second chromosome. In this study we investigated the genetic basis controlling the song differences associated with their sexual isolation, using intact and wingless males with chromosomes substituted between species. The results obtained from F₁ hybrid males between these species indicate the dominance of the song characters favoured by D. pallidosa females. In addition, the results obtained from backcross F₂ males indicate that chromosome 2 had a major effect on the control of the song characters associated with sexual isolation.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

Patterns of inheritance of mating signals in interspecific hybrids between sailfin and shortfin mollies (Poeciliidae: Poecilia: Mollienesia)

Differences in male morphology and mating behaviors are thought to confer species sexual isolation between sailfin and shortfin species of mollies. This study used interspecific crosses between the sailfin molly, P. latipinna, and the shortfin molly, P. mexicana, to investigate patterns of inheritance of morphological traits and behavioral rates of three mating behaviors in F1 hybrid males. The two parental species showed clear species differences with respect to the length of the dorsal fin and dorsal fin ray number. First generation hybrid males were intermediate between the two parental species for dorsal fin length and fin ray number, suggesting autosomal control of this trait with little effect of dominance by genes from either parental species. Parental species showed clear species differences in their rates of courtship displays. Unlike the pattern for dorsal fin morphology, F1 hybrid males showed a clear distinction in display rates with respect to the direction of the interspecific cross. Male hybrids whose sires were P. latipinna had courtship display rates that were up to three times higher than the rates of displays performed by hybrid males whose fathers were P. mexicana. The distribution of phenotypes between the parental species and that of hybrid males sired by that parental species was nearly identical. Such a pattern suggests the influence of Y-linked genes on the inheritance of courtship display rates in mollies.     

Behavioural mechanisms of sexual isolation involving multiple modalities and their inheritance

Speciation research dissects the genetics and evolution of reproductive barriers between parental species. Hybrids are the “gatekeepers” of gene flow, so it is also important to understand the behavioural mechanisms and genetics of any potential isolation from their parental species. We tested the role of multiple behavioural barriers in reproductive isolation among closely related field crickets and their hybrids (Teleogryllus oceanicus and Teleogryllus commodus). These species hybridize in the laboratory, but the behaviour of hybrids is unusual and there is little evidence for gene flow in the wild. We found that heterospecific pairs exhibited reduced rates of courtship behaviour due to discrimination by both sexes, and that this behavioural isolation was symmetrical. However, hybrids were not sexually selected against and exhibited high rates of courtship behaviour even though hybrid females are sterile. Using reciprocal hybrid crosses, we characterized patterns of interspecific divergence and inheritance in key sexual traits that might underlie the mating patterns we found: calling song, courtship song and cuticular hydrocarbons (CHCs). Song traits exhibited both sex linkage and transgressive segregation, whereas CHCs exhibited only the latter. Calculations of the strength of isolation exerted by these sexual traits suggest that close‐range signals are as important as long‐distance signals in contributing to interspecific sexual isolation. The surprisingly weak mating barriers observed between hybrids and parental species highlight the need to examine reproductive isolating mechanisms and their genetic bases across different potential stages of introgressive hybridization.     

From fruitless to sex: On the generation and diversification of an innate behavior

Male sexual behavior in Drosophila melanogaster, largely controlled by the fruitless (fru) gene encoding the male specific Fru^M protein, is among the best studied animal behaviors. Although substantial studies suggest that Fru^M specifies a neuronal circuitry governing all aspects of male sexual behaviors, recent findings show that Fru^M is not absolutely necessary for such behaviors. We propose that another regulatory gene doublesex encoding the male-specific Dsx^M protein builds a core neuronal circuitry that possesses the potential for courtship, which could be either induced through adult social experience or innately manifested during development by Fru^M expression in a broader neuronal circuitry. Fru^M expression levels and patterns determine the modes of courtship behavior from innate heterosexual, homosexual, bisexual, to learned courtship. We discuss how Fru^M expression is regulated by hormones and social experiences and tunes functional flexibility of the sex circuitry. We propose that regulatory genes hierarchically build the potential for innate and learned aspects of courtship behaviors, and expression changes of these regulatory genes among different individuals and species with different social experiences ultimately lead to behavioral diversification.     

POLYGENIC INHERITANCE OF A BEHAVIORAL PHENOTYPE: INTERSPECIFIC GENETICS OF SONG IN THE HAWAIIAN CRICKET GENUS LAUPALA

The Hawaiian cricket genus Laupala (family Gryllidae) is one of several native genera of flightless crickets found in rain-forest habitat across the Hawaiian archipelago. Species in this genus are morphologically quite similar, but the songs produced by adult males are acoustically distinct. I examined the nature of song variation found within Laupala paranigra and between Laupala kohalensis and L. paranigra, both endemic to the island of Hawaii. Variation within and among species was most notable in the temporal structure of the song, as quantified by the pulse rate. The variation in pulse rate present in natural populations of L. paranigra bred true through the F₁ laboratory generation, suggesting that the intraspecific variation in this species has a genetic basis. Interspecific hybridizations between L. kohalensis and L. paranigra successfully produced F₁, F₂, and backcross generations. Hybrid F₁ males from reciprocal crosses sang with significantly different pulse rates, implicating an X chromosomal contribution to the phenotypic difference between these species. Interspecific patterns of inheritance are most consistent with a type-I genetic architecture. Polygenic inheritance of the interspecific pulse-rate variation was observed, and approximately eight genetic factors were estimated to underlie the difference in pulse rate between L. kohalensis and L. paranigra.     

Geographic patterns of postzygotic isolation between two closely related widespread dung fly species (Sepsis cynipsea and Sepsis neocynipsea; Diptera: Sepsidae)

Identifying the contribution of pre‐ and postzygotic barriers to gene flow is a key goal of speciation research. The widespread dung fly species Sepsis cynipsea and Sepsis neocynipsea offer great potential for studying the speciation process over a range of opportunities for gene exchange within and across sister species (cross‐continental allopatry, continental parapatry and sympatry). We examined the role of postcopulatory isolating barriers by comparing female fecundity and egg‐to‐adult viability of F₁ and F₂ hybrids, as well as backcrosses of F₁ hybrids with the parental species, via replicated crosses of sym‐, para‐ and allopatric populations. Egg‐to‐adult viability was strongly but not totally suppressed in hybrids, and offspring production approached nil in the F₂ generation (hybrid breakdown), indicating yet unspecified intrinsic incompatibilities. Viable F₁ hybrid offspring showed almost absolute male (the heterogametic sex) sterility while females remained largely fertile, in accordance with Haldane's rule. Hybridization between the two species in European areas of sympatry (Swiss Alps) indicated only minor reinforcement based on fecundity traits. Crossing geographically isolated European and North American S. neocynipsea showed similar albeit weaker isolating barriers that are most easily explained by random genetic drift. We conclude that in this system with a biogeographic continuum of reproductive barriers, speciation is mediated primarily by genetic drift following dispersal of flies over a wide (allopatric) geographic range, with some role of natural or sexual selection in incidental or direct reinforcement of incompatibility mechanisms in areas of European sympatry. S(ubs)pecies status of continental S. neocynipsea appears warranted.     

GENETIC ARCHITECTURE OF ISOLATION BETWEEN TWO SPECIES OF SILENE WITH SEX CHROMOSOMES AND HALDANE'S RULE

Examination of the genetic architecture of hybrid breakdown can provide insight into the genetic mechanisms of commonly observed isolating phenomena such as Haldane's rule. We used line‐cross analysis to dissect the genetic architecture of divergence between two plant species that exhibit Haldane's rule for male sterility and rarity, Silene latifolia and Silene diclinis. We made 15 types of crosses, including reciprocal F₁, F₂, backcrosses, and later‐generation crosses, grew the seeds to flowering, and measured the number of viable ovules, proportion of viable pollen, and sex ratio. Typically, Haldane's rule for male rarity in XY animal hybrids is explained by interactions involving recessive X‐linked alleles that are deleterious when hemizygous (dominance theory), whereas sterility is explained by rapid evolution of spermatogenesis genes (faster‐male evolution). In contrast, we found that the genetic mechanisms underlying Haldane's rule between the two Silene species did not follow these conventions. Dominance theory was sufficient to explain male sterility, but male rarity likely involved faster‐male evolution. We also found an effect of the neo‐sex chromosomes of S. diclinis on the extreme rarity of some hybrid males. Our findings suggest that the genetic architecture of Haldane's rule in dioecious plants may differ from those commonly found in animals.     

GENETICS OF SEXUAL ISOLATION AND COURTSHIP DYSFUNCTION IN MALE HYBRIDS OF DROSOPHILA PSEUDOOBSCURA AND DROSOPHILA PERSIMILIS

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this “courtship dysfunction.” The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.     

FUNCTIONAL ASPECTS OF DROSOPHILA COURTSHIP

1. The elements that make up the courtship behaviour of males and of females are briefly described. It is pointed out that some of the terms used, such as female ‘repelling’ behaviour, are misleading as they do not reflect the known functions of the behaviours. 2. Evidence has been presented for a number of distinct pheromones with different functions during courtship. These claims are critically examined as the evidence is incomplete and at times conflicting. It seems unlikely that any pheromones other than those acting over a very short distance are involved in courtship. There is sound evidence for an aphrodisiac pheromone produced by all females which stimulates male courtship. A pheromone, which may be the same one, is produced by males less than 12 h old, which also stimulates male courtship. No function is ascribed to this pheromone. Fertilized females either produce less aphrodisiac pheromone or they may, in addition, produce one that inhibits male courtship. Mature males may also produce an inhibitory pheromone. Females produce a contact pheromone which is species-specific and involved in sexual isolation. It is not at present clear whether this is different from the aphrodisiac pheromone. 3. There is considerable variability in the importance of vision in courtship. Many species will mate satisfactorily in the dark, suggesting that visual stimuli are not critical. Most species use vision to orient towards one another and for males to track and follow females. Even in light-independent species such as D. melanogaster, specific visual signals may be used in courtship although they are not obligatory. Thus the red eye of the male is a sexual signal for females. Conversely, some light-dependent species do not appear to make use of visual signals as a major factor in courtship. Some, however, do perform behaviours that are clearly visual and which may act to emphasize markings on wings, head or body. 4. The majority of Drosophila species perform courtship songs by vibrating one or both wings. The songs produced by males sexually stimulate the females. They are species specific and there is considerable indirect and some direct evidence that the songs are involved in sexual isolation. Males of many species produce two different songs during courtship and it is probable that one is concerned mainly with sexual stimulation and the other with species recognition. Females of certain species of Drosophila and Zaprionus also sing during courtship and these songs may aid species recognition by males. In addition males and unreceptive females perform ‘aggressive’ songs. 5. Almost all studies of Drosophila courtship have been made in very confined conditions in the laboratory. Interpretation of some of the results obtained in this way may require modification in the light of ecological research and observation of courtships under more natural conditions.     

A comparison of the courtship and mating behavior of three species ofNasonia (Hymenoptera: Pteromalidae)

The parasitic waspNasonia vitripennis (Hymenoptera: Pteromalidae) is the subject of numerous genetic, evolutionary, ecological, and ethological studies, particularly relating to sex-ratio evolution, non-Mendelian inheritance, courtship behavior, and speciation. Here we describe the courtship behavior of two sibling species(N. longicornis andN. giraulti) and compare their courtship to that ofN. vitripennis. Courtship behavior ofNasonia males includes mounting of females, orientation to the female's head, and a repeated series of stereotypic movements, including head nods, mouthpart extrusions, antennal sweeps, and wing vibrations. Females signal receptivity by lowering their antennae in synchrony with opening their genital orifice. After copulation, males engage in brief postcopulatory displays. All three species have the basic components described above. However, although the three species are quite similar morphologically, there are distinct differences in courtship displays. Notable differences include the length of courtship cycles, the absence of a ritualized antennal sweep prior to the first cycle invitripennis, additional head nods and ritualized foreleg movements inlongicornis, and a dramatic increase in numbers of head nods ingiraulti. Results show that closely related species can often be distinguished based on courtship differences, although many of these differences may not contribute to a reproductive barrier.     

EVIDENCE FOR WIDESPREAD COURTSHIP DURING COPULATION IN 131 SPECIES OF INSECTS AND SPIDERS,AND IMPLICATIONS FOR CRYPTIC FEMALE CHOICE

Male courtship behavior is generally thought to function prior to copulation, as an inducement to the female to allow the male to copulate with her; this study indicates however, that male courtship during and following copulation (“copulatory courtship”) is common in insects and spiders (81% of 131 species in 102 genera and 49 families, mostly Coleoptera, Hemiptera, Diptera, and Araneioidea). Copulatory courtship is apparently evolutionarily labile, as expected if it is under sexual selection; intrageneric variation occurred in all 17 genera in which more than one species was observed. In 81% of 94 species with copulatory courtship, the male abandoned the female soon after copulation ended; thus, copulatory courtship appears not to function generally to induce acceptance of further copulatory attempts. The most likely explanation for copulatory courtship is that it represents attempts by males to influence cryptic female choice. This suggests that an aspect of sexual selection by female choice not considered by Darwin may be more important than previously appreciated and that the common practice in evolutionary studies of measuring male reproductive success by counting numbers of copulations may sometimes be misleading because of cryptic female choice during and after copulation.     

Sexual dimorphism and courtship behavior in Drosophila prolongata

Sexual dimorphism is often derived from sexual selection. In sexually dimorphic Drosophila species, exaggerated male structures are used for specific behaviors in male-to-male competition or courtship toward females. In Drosophila prolongata, a member of the melanogaster species group, males have enlarged forelegs whereas females do not. However, the adaptive role of the enlarged forelegs is unclear because little is known about the behavior of D. prolongata. In this study, the courtship behavior of D. prolongata was investigated in comparison with closely related species. Males of D. prolongata use their forelegs in a specific behavior, “leg vibration”, in which the male vigorously vibrates the female’s abdomen by extending his forelegs from in front of her. Leg vibration was observed immediately before “attempting copulation”, indicating that it has an adaptive role in the mating process. In contrast, leg vibration was not observed in closely related species. Because the large forelegs are necessary to accomplish leg vibration, it was suggested that the sexual dimorphism of D. prolongata forelegs is currently under the influence of sexual selection in courtship behavior.     

Six cryptic species on a single species of host plant: morphometric evidence for possible reproductive character displacement

Abstract 1. Diversification of some highly host‐specific herbivorous insects may occur in allopatry, without shifts in host use. Such allopatric divergence may be accelerated by sexual selection operating on courtship displays. Wing size and shape may affect visual and vibrational courtship displays in tephritid fruit flies. Geometric morphometric methods were used to examine wings of six sympatric cryptic species in the neotropical genus Blepharoneura. All six species feed on flowers of the same species of host (Gurania spinulosa), a neotropical vine in the Cucurbitaceae. Three of the fly species court and mate in close proximity on the host. Thus, courtship behaviours could serve as important reproductive isolating mechanisms. Two sets of hypotheses were tested: (i) species differ in wing shape and wing size; and (ii) species are sexually dimorphic in wing size and wing shape. Wing size differed among a few species, but wing shape differed significantly among all six species. Sexual dimorphism in wing size was found in only one species, but sexual dimorphism in wing shape was found in two of the three species known to court on the same host plant. In the two sexually dimorphic species, wing shape differed among males, but not among females. This suggests that selection for reproductive character displacement might accelerate divergence in wing shape.     

Singing on the wings! Male wing fanning performances affect female willingness to copulate in the aphid parasitoid Lysiphlebus testaceipes (Hymenoptera: Braconidae: Aphidiinae)

Lysiphlebus testaceipes (Hymenoptera: Braconidae: Aphidiinae) is a generalist endoparasitoid attacking more than 100 aphid species. In L. testaceipes, wing fanning is a main male courtship display evoked by a female‐borne sex pheromone. However, no information is available on the characteristics and behavioral role of male fanning during courtship in this parasitoid. Here, the courtship behavior of a wild strain of L. testaceipes was quantified and the male wing fanning performances were analyzed through high‐speed video recordings and examined in relation to mating success. Courtship sequence of wild L. testaceipes did not substantially differ from that previously reported for other populations mass reared on aphids. We observed that the male courtship duration did not affect mating success. However, video analysis revealed that the males producing high‐frequency fanning signals achieved higher mating success over those that display low‐frequency fanning. Wing fanning before successful and unsuccessful courtship differed in amplitude of wing movements and alignment toward the mate, highlighting that frontal courtship positively influence the female mating decisions. This study increases knowledge on sexual behavior in a key parasitoid of aphids, highlighting the importance of wing fanning among the range of sensory modalities used in the sexual communication of L. testaceipes. From a practical point of view, this information is useful in L. testaceipes‐based biocontrol strategies, since it can help to establish parameters for quality checking of mass‐reared wasps over time.     

The Role of Courtship Behavior and Size in Mate Preference in the Sex‐Role‐Reversed Gulf Pipefish,Syngnathus scovelli

In sex‐role‐reversed species, sexual selection acts more strongly on females than on males, a situation that can result in the evolution of secondary sexual traits in females and strong mating preferences in males. While some research exploring mating preferences in sex‐role‐reversed species has been conducted, overall, this topic remains relatively unexplored. The Gulf pipefish, Syngnathus scovelli, is a highly polyandrous pipefish species. Sexual selection is significantly stronger in females than in males, which has led to the evolution of both morphological and behavioral female secondary sexual traits. However, because males gestate the offspring in specialized pouches and make a substantial investment in embryos during development, females may also benefit from being choosy. The goal of this study was to examine both male and female mating preferences in this species. We found that male mating preference was significantly associated with female courtship behavior. Larger females were also able to maintain these behaviors for longer intervals than smaller females. No evidence of female mating preference in regard to male size was observed but the data suggest that male behaviors may be providing positive reinforcement to courting females. This research provides further insight into how mate preferences vary among sex‐role‐reversed species.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus (Orthoptera: Gomphocerinae): female preferences for courtship songs

Two grasshopper species, Stenobothrus rubicundus and Stenobothrus clavatus, were previously shown to hybridize in a narrow contact zone on Mount Tomaros in northern Greece. The species are characterized by complex and completely different courtship songs. In the present study, we investigated female preferences for the courtship songs of S. rubicundus, S. clavatus and hybrids in playback experiments. Playback of the courtship songs revealed assortative preferences in females of the parental species: they significantly more often preferred the songs of conspecific males. Hybrid females showed a lower selectivity than parental females, responding somewhat equally eager to playback of the songs of S. clavatus, S. rubicundus, and natural hybrid song, although less actively to the F₁ hybrid song. The results suggest that hybrid males may lose to males of parental species, whereas hybrid females would even have an advantage over parental females. Comparison of responses of females from allopatric populations and Mount Tomaros to different song types shows no evidence for reinforcement. Asymmetry found in female preferences may have implications for the structure of the hybrid zone. © 2013 The Linnean Society of London     

Reproductive isolation inProkelisia planthoppers (Homoptera: Delphacidae): Acoustic differentiation and hybridization failure

Males and females of Prokelisia marginata (Van Duzee) and Prokelisia dolusWilson communicate through substrate-transmitted vibrations. The acoustic signals (attraction and courtship calls) of these planthoppers are effective in mate location, attraction, and mate choice. Attraction calls are structurally distinct for both species and differ in pulse type, pulse repetition rate, and pulse duration. Using playback of prerecorded calls, individuals discriminated between conspecific and heterospecific signals. Depending on the sex and species, response calls were produced three to eight times more frequently to conspecifics than to heterospecifics. However, acoustic signals alone did not explain reproductive isolation and hybridization failure in these two congeners. Some heterospecific pairs called, courted, and attempted to join genitalia, but no connections were successful and no progeny were produced. Thus, acoustic behavior is not a guaranteed premating isolating mechanism in no-choice situations. Other courtship behaviors and possibly morphological differences in genitalia also contributed to their isolation. Females displayed a variety of rejection behaviors to conspecific and heterospecific males, suggesting that sexual selection (female choice), in addition to species recognition, may be an important force in the evolution of the acoustic signals of planthoppers. Although signal structure was not dependent on wing form (planthoppers exhibit wing dimorphism), the age when females first began to call was related to wing form. Brachypterous (flightless) females of both species began calling early in adult life (day 2), whereas macropterous (migratory) females began calling later in adult life (day 6). This pattern is consistent with the oogenesis-flight syndrome, in which reproductive maturity is delayed until after migration occurs.     

SEX CHROMOSOME LINKAGE OF MATE PREFERENCE AND COLOR SIGNAL MAINTAINS ASSORTATIVE MATING BETWEEN INTERBREEDING FINCH MORPHS

Assortative mating is a key aspect in the speciation process because it is important for both initial divergence and maintenance of distinct species. However, it remains a challenge to explain how assortative mating evolves when diverging populations are undergoing gene flow (e.g., during hybridization). Here I experimentally test how assortative mating is maintained with frequent gene flow between diverged head‐color morphs of the Gouldian finch (Erythrura gouldiae). Contrary to the predominant view on the development of sexual preferences in birds, cross‐fostered offspring did not imprint on the phenotype of their conspecific (red or black morphs) or heterospecific (Bengalese finch) foster parents. Instead, the mating preferences of F₁ and F₂ intermorph‐hybrids are consistent with inheritance on the Z chromosomes, which are also the location for genes controlling color expression and the genes causing low fitness of intermorph‐hybrids. Genetic associations between color signal and preference loci on the sex chromosomes may prevent recombination from breaking down these associations when the morphs interbreed, helping to maintain assortative mating in the face of gene flow. Although sex linkage of reproductively isolating traits is theoretically expected to promote speciation, social and ecological constraints may enforce frequent interbreeding between the morphs, thus preventing complete reproductive isolation.