Insect hygroreceptor responses to continuous changes in humidity and air pressure.

The most favored model of humidity transduction views the cuticular wall of insect hygroreceptive sensilla as a hygromechanical transducer. Hygroscopic swelling or shrinking alters the geometry of the wall, deforming the dendritic membranes of the moist and dry cells. The small size the sensilla and their position surrounded by elevated structures creates technical difficulties to mechanically stimulate them by direct contact. The present study investigated hygroreceptors on the antennae of the cockroach and the stick insect. Accurately controlled, homogeneous mechanical input was delivered by modulating air pressure. Both the moist and dry cells responded not only to changes in air pressure but also in the opposite direction, as observed during changes in air humidity. The moist cell's excitatory response to increasing humidity and increasing air pressure implies that swelling of the hygroscopic cuticle compresses the dendrites, and the dry cell's excitatory response to decreasing humidity and decreasing air pressure implies that shrinking of the hygroscopic cuticle expands the dendrites. The moist and dry cells of the stick insect are more sensitive to pressure changes than those of the cockroach, but the responses to air pressure are generally weaker than to humidity. Therefore the hygroreceptive sensilla differ in their physical properties and constitutions. Furthermore, the mechanical parameters associated with homogeneous changes in air pressure on the sensillum surface can only partially account for the responses of the moist and dry cells of both species to humidity stimulation.

Continuous tonic spike activity in spider warm cells in the absence of sensory input.

The warm cells of the spider tarsal organ respond very sensitively to low-amplitude changes in temperature and discharge continuously as the rate of change in temperature reaches zero. To test whether the continuous tonic discharge remains without sensory input, we blocked the warm cell's receptive region by Epoxy glue. The activity continued in this situation, but its dependence on temperature changes was strongly reduced. We interpret this to mean that the warm cells exhibit specific intrinsic properties that underlie the generation of the tonic discharge. Experiments with electrical stimulation confirmed the observation that the warm cells persist in activity without an external drive. In warm cells with blocked receptive region, the response curves describing the relationship between the tonic discharge and the level of depolarization is the same for different temperatures. In warm cells with intact receptive region, the curves are shifted upward with rising temperature, as if the injected current is simply added to the receptor current. This indicates a modulating effect of the receptor current on the tonic discharge. Stimulation causes a change in the tonic discharge rate and thereby enables individual warm cells to signal the direction in addition to the magnitude of temperature changes.

Low rates of change enhance effect of humidity on the activity of insect hygroreceptors.

The inability to measure humidity during stimulation has so far prevented us from understanding the contribution of moist cells and dry cells to orientation in a gradient of humidity. The problem was solved in the present study by means of a UV-absorption hygrometer that made it possible to monitor humidity at a rate of 100 Hz. The antennal moist and dry cells of the cockroach were exposed to humidities alternatively falling or rising at low rates between -1% RH s(-1) and +1% RH s(-1) (relative humidity). Impulse frequency of both types of cells depended simultaneously on instantaneous humidity and its rate of change. High frequencies of the moist cells signal high humidity. But at a given humidity, the response frequency is higher still when humidity is also rising. Conversely, high frequencies of the dry cell signal low humidity, and frequency is higher still at a given humidity when humidity is also falling. These responses ensure that the cockroach spent a minimum time in environments where desiccation or hydration occur and may thus protect the animal from emerging accidentally from under cover into moving air. In the constant-humidity retreat of the cockroach, fluctuating or even drifting discharge frequencies could serve as an early warning: return!

Residual impact of the biocontrol inoculant Pseudomonas fluorescens F113 on the resident population of rhizobia nodulating a red clover rotation crop.

A field trial was previously conducted in which sugarbeet seeds were either untreated, inoculated with the biocontrol strain Pseudomonas fluorescens F113Rif, or treated with chemical fungicides. Following harvest of sugarbeet, the field site was sown with uninoculated red clover. The aim of this study was to assess the residual impact of the microbial inoculant (and the fungicide treatment) on the diversity of resident rhizobia nodulating the red clover rotation crop. The percentage of nodules yielding rhizobial isolates after surface disinfection was 67% in the control and 70% in the P. fluorescens F113Rif treatment, but only 23% in the chemical treatment. Isolates were characterized by RAPD analysis. The main RAPD cluster (arbitrarily defined at 70% similarity) was prevalent in all three treatments. In addition, the distribution of RAPD clusters followed a log series model, regardless of the treatment applied, indicating that neither the microbial inoculant nor the fungicide treatment had caused a strong perturbation of the rhizobial population. When the P. fluorescens F113Rif and control treatments were compared using diversity indices, however, it appeared that the genetic diversity of rhizobia was significantly less in the inoculated treatment. The percentage of rhizobia sensitive to 2,4-diacetylphloroglucinol (Phl; the antimicrobial metabolite produced by P. fluorescens F113Rif) fluctuated according to field site heterogeneity, and treatments had no effect on this percentage. Yet, the proportion of Phl-sensitive isolates in the main RAPD cluster was lower in the P. fluorescens F113Rif treatment compared with the control, raising the possibility that the residual impact of the inoculant could have been partly mediated by production of Phl. This impact on the rhizobial population took place without affecting the functioning of the Rhizobium-clover symbiosis.

Cold-receptor cells supply both cold- and warm-responsive projection neurons in the antennal lobe of the cockroach.

The temperature receptor cells on the cockroach antennae are all excited by rapid cooling. In the antennal lobe, however, cold- as well as warm-responsive neurons occur. They are excited either by rapid step-like cooling or rapid step-like warming. Responses to such temperature transients do not show, however, whether antennal lobe neurons convey information on slowly changing temperatures typical of temperature gradients used for orientation. In contrast slow temperature changes permit an analysis of the effects of both instantaneous temperature and its rate of change. We compared the effect of slow temperature oscillations on the responses of antennal cold-receptors cells and cold- and warm-responsive projection neurons. In all cases the discharge rates were modulated by the temperature oscillations. They displayed a double dependence on instantaneous temperature and its rate of change. Information about cooling and warming, first contained in the output of a single cold-receptor cell diverges to form the parallel pathways of cold- and warm-responsive projection neurons, thereby in particular improving the detection of fluctuations in temperature.

Identification of two Ikaros-like transcription factors in lamprey.

The jawless Agnatha (lampreys and hagfishes) represent the phylogenetically oldest order of vertebrates that are believed to lack the adaptive immune system of jawed vertebrates. In order to search for molecular markers specific for cellular components of the adaptive immune system in lampreys, we used the polymerase chain reaction (PCR) to identify genes for transcription factors of the Ikaros family in genomic DNA and cDNA libraries from two species of lampreys, Petromyzon marinus and Lampetra fluviatilis. The mammalian Ikaros-like family of transcription factors consists of five members, Ikaros, Helios, Aiolos, Eos and Pegasus, of which the first three appear to be essential for lymphocyte development. Two different Ikaros-like genes, named IKLF1 and IKLF2, were identified in lamprey. They both have the conserved exon-intron structure of seven exons and show alternative splicing like their counterparts in jawed vertebrates. The genes code for predicted proteins of 589 and 513 amino acid residues, respectively. The proteins contain six highly conserved zinc finger motifs that are 83-91% identical to the mammalian members of the Ikaros-like family. The remaining parts of the sequences are, however, mostly unalignable. Phylogenetic analysis based on the alignable segments of the sequences does not identify the orthologous gene in jawed vertebrates but rather shows equidistance of the lamprey Ikaros-like factors to each other and to Ikaros, Helios, Aiolos and Eos. Expression studies by reverse transcription (RT)-PCR and in situ hybridization (ISH), however, provide evidence for moderate expression in presumed lymphoid tissues like the gut epithelium and for high levels of expression in the gonads, especially in the ovary.

Infrared sensitivity of thermoreceptors.

This study compares the effects of convective and radiant heat on the discharge rates of the warm cell of a thin hair-like sensillum of the tick and of the cold cells of small peg-shaped sensilla of the locust and the cockroach. The temperature rates imposed by the convective heat contained in the air stream used for stimulation are reflected by the discharge rate of the thermoreceptors. We determined the increment in radiant heat that results in the same change in discharge rate as a given increment in temperature due to convection. The amount of infrared radiation required to produce the same effect as a 1 degrees C change in temperature differs for the sensory cells of the tick, locust and cockroach, respectively, suggesting differences in the ability of the sensilla to take up and transfer radiant heat. The power of radiation required to modulate the discharge rates is very high and outside the biologically meaningful range in all cases. Obviously the adequate stimulus for the examined sensilla is convective heat and not radiant heat.

Classification and phylogenetic relationships of African tilapiine fishes inferred from mitochondrial DNA sequences.

African cichlid fishes are composed of two major lineages, the haplochromines and the tilapiines. Whereas the phylogenetic relationships of the haplochromines have been studied extensively, primarily because of their spectacular adaptive radiations in the Great Lakes of East Africa, little is known about the relationships among the tilapiine species, despite the fact that they have become an important component of African, indeed world, aquaculture. To remedy this situation, molecular phylogenetic analysis of tilapiine fishes was undertaken. A segment of mitochondrial DNA encompassing the terminal part of the tRNA(Pro) gene and the most variable part of the control region was amplified by the polymerase chain reaction with DNA samples isolated from 42 tilapiine species, and the amplification products were subjected to heteroduplex analysis and sequencing. Phylogenetic trees based on 68 sequences revealed the existence of 11 sequence groups and 11 single-sequence branches. The groups, designated Ti1 through Ti11, were distinguished by specific combinations of diagnostic substitutions, formation of monophyletic clusters, and separation by genetic distances in excess of 0.04. Although the relationships among the groups could not be resolved, the sequences separated Oreochromis and Sarotherodon from Tilapia, as defined by Trewavas. The Oreochromis sequences clustered with the Sarotherodon sequences and thus supported the hypothesis that the mouthbrooding behavior of the tilapiine fishes evolved only once from the substrate-spawning behavior. Since on phylogenetic trees the O. alcalicus (sub)species were always separated from O. amphimelas by other Oreochromis species, it was concluded that the adaptation to life in water with a high salt concentration and high pH values evolved independently at least twice in the tilapiine fishes. The tilapiines diverged from the haplochromines more than 8 million years ago; most of the intragroup divergences among the tilapiines took place an estimated 1.1 to 6 million years ago.

Comparative strain typing of Rhizobium leguminosarum bv. viciae natural populations.

372 natural isolates of Rhizobium leguminosarum bv. viciae, rescued from nodules of pea plants grown in an agricultural field in northern Italy, were analyzed by different methods. Three DNA-based fingerprinting techniques were lined up to compare their relative degree of resolution and possible advantages of each approach. The methods included (i) Eckhardt gel plasmid profiles, (ii) pulsed-field gel electrophoresis (PFGE) of genomic large fragment digests, and (iii) random amplified polymorphic DNA (RAPD) profiles, generated with arbitrary primers. The scheme also involved the isolation of a number of different isolates per nodule to estimate the level of intra-nodular variability. It was therefore possible to evaluate the frequency of double and multiple occupancies, and the proportion of the alternative profiles sharing the same nodule, generally resulting in a numerically dominant, main representative accompanied by a secondary one with a slightly different fingerprint. This finding revealed that the different profiles within a nodule are normally due to bacteria derived from the same single invader following genetic alterations possibly occurred during infection, e.g., by plasmid loss. The analysis of 31 nodules revealed 16 different patterns, representing the most frequently occurring nodulation-proficient isolates of the natural soil examined, five of which were found with frequencies around 15%. The sensitivity of the methods in differentiating isolates was compared. The relatedness of the different natural rhizobial isolates was investigated by densitometrical gel analysis of the fingerprints, allowing a comparison of the results. One of the most interesting conclusions was that the degree of information yielded by the plasmid gel profiling alone, carried out by simple visual inspection without software-aided analyses, was surprisingly high, as it enabled a placement of the isolates, whose accuracy, in terms of relatedness, was subsequently confirmed by each of the two genomic methods.

Impact of 2,4-diacetylphloroglucinol-producing biocontrol strain Pseudomonas fluorescens F113 on intraspecific diversity of resident culturable fluorescent pseudomonads associated with the roots of field-grown sugar beet seedlings.

The impact of the 2,4-diacetylphloroglucinol-producing biocontrol agent Pseudomonas fluorescens F113Rif on the diversity of the resident community of culturable fluorescent pseudomonads associated with the roots of field-grown sugar beet seedlings was evaluated. At 19 days after sowing, the seed inoculant F113Rif had replaced some of the resident culturable fluorescent pseudomonads at the rhizoplane but had no effect on the number of these bacteria in the rhizosphere. A total of 498 isolates of resident fluorescent pseudomonads were obtained and characterized by molecular means at the level of broad phylogenetic groups (by amplified ribosomal DNA restriction analysis) and at the strain level (with random amplified polymorphic DNA markers) as well as phenotypically (55 physiological tests). The introduced pseudomonad induced a major shift in the composition of the resident culturable fluorescent Pseudomonas community, as the percentage of rhizoplane isolates capable of growing on three carbon substrates (erythritol, adonitol, and L-tryptophan) not assimilated by the inoculant was increased from less than 10% to more than 40%. However, the pseudomonads selected did not display enhanced resistance to 2,4-diacetylphloroglucinol. The shift in the resident populations, which was spatially limited to the surface of the root (i.e., the rhizoplane), took place without affecting the relative proportions of phylogenetic groups or the high level of strain diversity of the resident culturable fluorescent Pseudomonas community. These results suggest that the root-associated Pseudomonas community of sugar beet seedlings is resilient to the perturbation that may be caused by a taxonomically related inoculant.

Female sex pheromone of a wandering spider (Cupiennius salei): identification and sensory reception.

Females of the wandering spider Cupiennius salei attach a sex pheromone to their dragline. Males encountering the female dragline examine the silk thread with their pedipalps and then typically initiate reciprocal vibratory courtship with the sexual partner. The female pheromone was identified as (S)-1,1'-dimethyl citrate. The male pheromone receptive sensory cells are located in tip pore sensilla and respond to touching the sensillum tip with female silk or pieces of filter paper containing the synthetic pheromone.

On the origin of Darwin's finches.

Darwin's finches comprise a group of 15 species endemic to the Galápagos (14 species) and Cocos (1 species) Islands in the Pacific Ocean. The group is monophyletic and originated from an ancestral species that reached the Galápagos Archipelago from Central or South America. Descendants of this ancestor on the Archipelago then colonized Cocos Island. In the present study, we used sequences of two mitochondrial (mt) DNA segments (922 bp of the cytochrome b gene and 1,082 bp of the control region), as well as two nuclear markers (830 bp of numt2, consisting of 140 bp of mtDNA control region and 690 bp of flanking nuclear DNA; and 740 bp of numt3, consisting of 420 bp of mt cytochrome b sequence flanked by 320 bp of nuclear DNA) to identify the species group most closely related to the Darwin's finches. To this end, we analyzed the sequences of 28 species representing the main groups (tribes) of the family Fringillidae, as well as 2 outgroup species and 13 species of Darwin's finches. In addition, we used mtDNA cytochrome b sequences of some 180 additional Fringillidae species from the database for phylogeny reconstruction by maximum-parsimony, maximum-likelihood, minimum-evolution, and neighbor-joining methods. The study identifies the grassquit genus Tiaris, and specifically the species Tiaris obscura, as the nearest living relative of Darwin's finches among the species surveyed. Darwin's finches diverged from the Tiaris group shortly after the various extant species of Tiaris diverged from one another. The initial adaptive radiation of the Tiaris group apparently occurred on the Caribbean islands and then spread to Central and South America, from where the ancestors of Darwin's finches departed for the Galápagos Islands approximately 2.3 MYA, at the time of the dramatic climatic changes associated with the closure of the Panamanian isthmus and the onset of Pleistocene glaciation.

Mhc class I genes of swordtail fishes, Xiphophorus: variation in the number of loci and existence of ancient gene families.

Swordtail fishes and platies in the genus Xiphophorus (order Cyprinodontiformes, Teleostei) encompass 22 closely related species which are the products of a recent adaptive radiation in the streams of Central America. To investigate the evolution of the major histocompatibility complex (Mhc) genes in the period immediately following speciation, the class I genes from 20 of the 22 species were cloned and characterized by sequencing. The analysis revealed the existence of multiple loci (at least seven in some individuals) whose numbers vary among the different species and probably also among individuals of the same species. The variation does not seem to bear any relationship to the taxonomy of the genus. Genes at the different loci are distinguished by their intron sequences and by the presence of characteristic motifs in exons 2 and 3. The variation in copy number of loci may have been effected in part by unequal crossing over occurring between introns of misaligned closely related genes. The sequences of the genes fall into two groups, A and B, which represent ancient lineages. The groups define two families of loci, which diverged from each other an estimated 85 million years ago, before the separation of the Acanthopterygii from the Paracanthopterygii of the advanced bony fishes. Evolution of the genes within each family can be explained by the birth-and-death process driven by gene duplications and mutational differentiation.

Evolution of Mhc class II B genes in Darwin's finches and their closest relatives: birth of a new gene.

The 15 extant species of Darwin's finches on the Galápagos and Cocos Islands are the products of an unfinished adaptive radiation from a founder flock of birds related to the South American species Tiaris obscura. Molecular characterization of their major histocompatibility complex ( Mhc) class II B genes has revealed the existence of several related groups of sequences (presumably encoded in distinct loci) from which one (group 5) stands out because of its low divergence over extended time periods. Analysis of group 5 exon 2 and intron 2 sequences has revealed that the encoding locus apparently arose 2-3 million years ago in the Tiaris group of South and Central American Thraupini. The locus shows no evidence of inactivation, but displays a very low degree of polymorphism, both in terms of number of alleles and genetic distances between alleles. Some of the polymorphism, however, appears to be trans-specific. All the observed intergenic differences can be explained by point mutations and most of the exon 2 changes represent non-synonymous substitutions, although the rate of non-synonymous and synonymous substitutions appears to be the same. The origin of the new locus is explained by the birth-and-death model of Mhc evolution with two important extensions. First, the ancestor of the group 5 genes may have arisen without new gene duplication and second, the birth of the new group may have been brought about by a switch from balancing to directional selection. The ancestor of the group 5 genes may have been a classical class II B allele (one of many) which directional selection fixed in the ancestral population and drove into the category of nonclassical genes.

Integration of temperature and olfactory information in cockroach antennal lobe glomeruli.

Individual neurons in the antennal lobe of the cockroach not only respond to warming, cooling and the odor of lemon oil but they also integrate the responses to simultaneously occurring temperature and olfactory stimuli. This integration results in an increase or decrease of the neuron's activity as compared to its responses to the temperature stimuli presented alone. The mean gain for a change in temperature in the warm and cold direction is 9.5 (imp s(-1)) degrees C(-1) and 10.2 (imp s(-1)) degrees C(-1), respectively. Thus, the average neuron elevates its impulse frequency by 1 imp s(-1) when temperature is increased by 0.1 degree C or decreased by 0.09 degree C. Examination of response scatter reveals that the difference required between two warm or two cold stimuli to be discriminated is 0.5 degree C. Similar values for gain and resolving power are obtained for the enhanced responses to the warm-odor and the cold-odor stimulus combinations. The neurons described are: (1) local interneurons innervating a number of glomeruli distributed within the antennal lobe, and (2) projection neurons collecting information from single glomeruli at 140-280 microm from the surface of the antennal lobe and providing links with the calyces of the mushroom bodies and the lateral lobe of the protocerebrum.

Mhc class II B gene evolution in East African cichlid fishes.

A distinctive feature of essential major histocompatibility complex (Mhc) loci is their polymorphism characterized by large genetic distances between alleles and long persistence times of allelic lineages. Since the lineages often span several successive speciations, we investigated the behavior of the Mhc alleles during or close to the speciation phase. We sequenced exon 2 of the class II B locus 4 from 232 East African cichlid fishes representing 32 related species. The divergence times of the (sub)species ranged from 6,000 to 8.4 million years. Two types of evolutionary analysis were used to elucidate the pattern of exon 2 sequence divergence. First, phylogenetic methods were applied to reconstruct the most likely evolutionary pathways leading from the last common ancestor of the set to the extant sequences, and to assess the probable mechanisms involved in allelic diversification. Second, pairwise comparisons of sequences were carried out to detect differences seemingly incompatible with origin by nonparallel point mutations. The analysis revealed point mutations to be the most important mechanism behind allelic divergences, with recombination playing only an auxiliary part. Comparison of sequences from related species revealed evidence of random allelic (lineage) losses apparently associated with speciation. Sharing of identical alleles could be demonstrated between species that diverged 2 million years ago. The phylogeny of the exon was incongruent with that of the flanking introns, indicating either a high degree of convergent evolution at the peptide-binding region-encoding sites, or intron homogenization.

Do lampreys have lymphocytes? The Spi evidence.

It is generally accepted that living jawless vertebrates (lampreys and hagfishes) lack the capability of mounting an adaptive immune response. At the same time, however, there are reports describing histological evidence for the presence in agnathan tissues of lymphocytes, the key players in adaptive immunity. The question therefore arises whether the cells identified morphologically as lymphocytes are true lymphocytes in terms of their genetic developmental program. In this study, evidence is provided that the lampreys express a member of the purine box 1 (PU.1)/spleen focus-forming virus integration B (Spi-B) gene family known to be critically and specifically involved in the differentiation of lymphocytes in jawed vertebrates. The lamprey gene is expressed in the lymphocyte-like cells of the digestive tract and inexplicably also in the ovary.

The origin and age of haplochromine fishes in Lake Victoria, east Africa.

According to a widely held view, the more than 300 species of haplochromine cichlid fishes in Lake Victoria (LV), East Africa, originated from a single founder species in less than 12,000 years. This view, however, does not follow from the published geological and molecular evidence. The former does indeed suggest that the LV basin dried out less than 15,000 years ago, but it does not provide any information about the species that re-colonized the new lake or that remained in the rivers draining the area. The molecular evidence is inconclusive with respect to the origin of the LV haplochromines because cichlids from critical regions around LV were not adequately sampled; and as far as the age of the LV haplochromines is concerned, it in fact led to an estimate of 250,000-750,000 years old. In the present study, mitochondrial DNA (control region) variation was determined by heteroduplex and sequencing analyses of more than 670 specimens collected at widely distributed East African riverine and lacustrine localities. The analyses revealed the existence of seven haplogroups (I-VII) distinguishable by characteristic substitutions. All endemic LV samples tested fell into one of these haplogroups (V) which, however, was also found to be present at various other localities, both riverine and lacustrine, outside LV. Within this haplogroup, four subgroups (VA through VD) could be distinguished, two of which (VB and VC) were represented in LV and at other localities. The great majority of the LV haplochromine species could be classified as belonging to the VC subgroup, which was found only in LV and in the rivers draining into it. Hence, while the endemic haplochromine species of LV could not have originated from a single founding population, the lake does harbour a large species flock which probably arose in situ.

Expression of the dentin matrix protein 1 gene in birds.

The emergence of jawed vertebrates was predicated on the appearance of several innovations, including tooth formation. The development of teeth requires the participation of several specialized genes, in particular, those necessary for the formation of hard tissues--dentin, enamel, and cementum. Some vertebrates, most conspicuously birds, secondarily lost the tooth-forming ability. To determine the fate of some of the tooth-forming genes in the birds, we tested a domestic fowl cDNA library for the expression of the dentin matrix protein 1 (DMP1) gene. The library was prepared from the poly(A+) RNA isolated from the jaws of 11- to 13-day-old embryos and the testing was carried out by the polymerase chain reaction with degenerate primers designed on the basis of the available mammalian and reptile sequences. A chicken homologue of the DMP1 gene identified by this approach was shown to be expressed in the jaws and long bones, the same two tissues as in mammals. The chicken DMP1 gene has an exon/ intron organization similar to that of its mammalian and reptile counterparts. The chicken gene contains three short highly conserved segments, the rest of the gene being poorly alignable or not alignable with its mammalian or reptilian homologues. The distribution of similarities and dissimilarities along the gene is indicative of a mode of evolution in which only short segments are kept constant, while the rest of the gene is relatively free to vary as long as the proportion of certain amino acid residues is retained in the encoded polypeptide. The DMP1 gene may have been retained in birds because of its involvement in bone formation.

Convergent evolution of major histocompatibility complex molecules in humans and New World monkeys.

In both Old World and New World monkeys Mhc-DRB sequences have been found which resemble human DRB1*03 and DRB3 genes in their second exon. The resemblance is shared sequence motifs and clustering of the genes or the encoded proteins in phylogenetic trees. This similarity could be due to common ancestry, convergence at the molecular level, or chance. To test which of these three explanations applies, we sequenced segments of New World monkey and macaque genes which encompass the entire second exon and large parts of both flanking introns. The test strongly supports the monophyly of New World monkey DRB intron sequences. The phylogenies of introns 1 and 2 from DRB1*03-like and DRB3-like genes are congruent, but both are incongruent with the exon 2-based phylogeny. The matching of intron 1- and intron 2-based phylogenies with each other suggests that reciprocal recombination has not played a major role in exon 2 evolution. Statistical comparisons of exon 2 from different DRB1*03 and DRB3 lineages indicate that it was neither gene conversion (descent), nor chance, but molecular convergence that has shaped their characteristic motifs. The demonstration of convergence in anthropoid Mhc-DRB genes has implications for the classification, age, and mechanism of generation of DRB allelic lineages.