A comparative study of the chromosomes in the incipient species of the Drosophila paulistorum complex

Summary Drosophila paulistorum Dobzhansky et Pavan is a complex of six races or incipient species. The races are mostly allopatric, but they are reproductively isolated sufficiently to permit them to exist also sympatrically in some places. The gene arrangements in the chromosomes of the races have been compared by means of examination of the giant chromosomes in the larval salivary glands; 28 strains of all races, and about an equal number of interracial hybrids have been studied.Chromosomal inversion polymorphism has been discovered in all races, even in the Guianan race of which only a single strain is available. Inversion heterozygotes are found in every one of the five chromosomal strands which the species has. Interracial hybrids tend to be heterozygous for more inversions than are present in the strains of the parental races. The Transitional race has however much the same gene arrangements as the widespread Andean — South Brazilian race.With the exception of the Transitional race, and of three other possible exceptions, each race has a collection of its own race-specific inversion polymorphs, not found in the other races. This very striking finding is discussed in connection with the hypothesis which envisages the origin of new species from marginal colonies at the periphery of the geographic distribution area of the ancestral species.The work reported in this article has been carried under Contract No. AT-(30-1)-1151, U.S. Atomic Energy Commission, mostly at the Department of Zoology, Columbia University, New York.     

A comparative study of the chromosomal polymorphs in the incipient species of the Drosophila paulistorum complex

Ninety-six geographical strains distributed among the incipient species of the Drosophila paulistorum complex were examined cytologically, and the results obtained were correlated with available data on hybridization tests and chromosomal analysis. The complex was found to contain more than sixty-three different inversions, out of which thirty-two were 3rd chromosome configurations. This placed Drosophila paulistorum among the most chromosomally polymorphic species in the genus. The species differs from D. willistoni, in that a great number of inversions is concentrated in one of the chromosomes, as opposed to approximately equal distribution of inversions in the chromosomes of willistoni. — The data obtained in the course of this investigation seem to support the idea that either massive populations become isolated and then form new species, or that the newly forming species tend to retain some of their ancestral polymorphs which might present them with heterotic effects, gradually replacing them with more successful combinations as speciation progresses.The work reported in this article has been carried under Contract No. AT (30-1)-3096, U. S. Atomic Energy Commission.     

UNUSUAL FILAMENTOUS STRUCTURES IN THE PARAGONIA OF MALE DROSOPHILA

Drosophila paulistorum is a complex of five incipient species which when crossed produce sterile hybrid males and fertile females. Sterility of the male progeny can sometimes be induced by injecting females of one strain (Mesitas) with a homogenate of males of another strain (Santa Marta) or of hybrids between these strains, and then crossing the recipient females to Mesitas males. Filamentous structures have been found in cytoplasmic vacuoles in paragonial cells in males of these and other similar strains and their hybrids. These structures, which contain RNA, possess a helical substructure and resemble certain viruses. Large filamentous structures found in the lumen of the paragonia are also described.     

Über die Resistenz von Solanum (Tuberarium)-Arten gegen europäische Rassen des Kartoffelnematoden (Heterodera rostochiensis Woll.)

Zusammenfassung o| li]1.|63 Arten mit 200 Herkünften und 2 kultivierte Formen von Solanum wurden mit der Rasse A und 11 davon abweichenden (B-) Rassen auf Cystenbildung getestet, unter dankenswerter Mitarbeit der Herren A. Bumbulucz, J. M. Dunnett, H. Goffart, J. Münster, L. Roer, D. Rothacker, A. Savary, H. Stelter und B. Weischer. li]2.|Es wurde für jede Rasse ein spezifischer Wirtskreis gefunden. li]3.|Die Rassen unterscheiden sich in der Prozentzahl der für sie anfälligen Wirtsarten und Herkünfte (Aggressivitätszahl). Je größer die Aggressivitätszahl einer Rasse ist, desto seltener wird ein mit ihr resistent reagierender Wirt von einer anderen Rasse befallen. Rassen mit hoher Aggressivitätszahl sind für die Auslese resistenter Ausgangsformen daher die günstigsten. li]4.|Die Arten S. oplocense, S. spegazzinii (syn. S. famatinae) und S. vernei sind gegen alle 11 und die Arten bzw. Herkünfte S. sanctae-rosae P.H. 328 und EBS 1778 und S. tuberosum Primitivform aus Chile EBS 2084 gegen alle 6 geprüften Rassen resistent.Gegen 9 Rassen resistent und gegen eine (A Wageningen) anfällig ist S. andreanum. Gegen 4–6 Rassen resistent und gegen eine anfällig sind S. megistacrolobum EBS 1783 und S. sparsipilum EBS 1801 und 1890. li]5.|Veröffentlichte und unveröffentlichte Untersuchungen haben gezeigt, daß die Resistenz durch Hauptgene vererbt wird, wobei ein Gen gegen mehrere der oben genannten Pathotypen wirken kann.Es erscheint sonach möglich, daß in den resistenten Solanum-Formen Gene vorkommen, die eine umfassende Pathotypenresistenz vererben. Auf diesen Genen wäre eine Resistenzzüchtung aufzubauen. li]6.|Die Reaktion der z. Z. in der Züchtung benutzten Bastarde resistente Wildart x S. tuberosum mit den untersuchten Rassen ist in Tab. 4 wiedergegeben.

---

On the resistance of species of Solanum (Tuberarium) against the European Races of the potato nematode (Heterodera rostochiensis Woll.)

Summary 63 tuber bearing Solanum species of 200 proveniencies were screened for cyst production with race A and 11 deviating (B–) races found in Scotland, Norway, The Netherlands, Germany, and Switzerland. The authors thankfully acknowledge the co-operation of A. Bumbulucz, J. M. Dunnett, H. Goffart, J. Münster, L. Roer, D. Rothacker, A. Savary, H. Stelter and B. Weischer.Each race could be characterized by a specific host range, which was different for every race. The races are distinguished by their aggressivity, expressed by the aggressivity number, i.e. the percentage of hosts among all the hosts tested that will form cysts. The race Chavornay has with 13,6% the lowest aggressivity number and the race ABC with 59,0% the highest. Solanum forms which are resistant to a race with high aggressivity number have the best chance to maintain this resistance against many races occurring in Europe.The following species are resistant to all 11 races: S. oplocense, S. spegazzinii (syn. S. famatinae) and S. vernei. The species S. sanctae-rosae EBS 1778 and P.H. 328, S. megistacrolobum EBS 1783, and S. tuberosum Chile EBS 2084 were tested with 6 races (the first two inclusive ABC) and found to be resistant to all. S. andreanum EBS 2183 and S. sparsipilum EBS 1801 and 1890 were resistant to 5–9 races but susceptible to one.Published and unpublished results showed the existence of major genes in resistant species responsible for resistance to one as well as to several races. It therefore seems possible to base breeding for resistance on a few major genes only.Some hybrids, wild species x S. tuberosum, are used in breeding work. The reaction of these hybrids with the different pathotypes is reproduced in table 4.

---

---

Frl. Marianne Benter danken wir für ihre ausgezeichnete Assistenz.     

Population genetics in the american tropics. VIII. The courtship isolation within Drosophila paulistorum incipient “species”

The so called races of Drosophila paulistorum from South America are so deeply isolated ethologically that although still partially bridged by gene flow through hybridization, they should be considered as 6 separate sibling species just as the willistoni group to which D. paulistorum itself belongs. The present article emphasizes the way in which isolation is implemented.This paper is dedicated to him on his 72nd birthday for his many contributions to our field.     

The significance of biological races of Ditylenchus dipsaci and their hybrids

Pure cultures of six races of Ditylenchus dipsaci derived from a single female, namely, lucerne race (LR), red clover race (RCR), white clover race (WCR), narcissus race (NR), tulip race (TR) and oat race (OR) were inoculated into eight plant species growing in pots. Onion was host to all six races, and tulip to all except RCR, whereas lucerne was susceptible only to LR and slightly to WCR. Hyacinth was not very susceptible to any race. The TR and OR were the most polyphagous. Severe symptoms were not necessarily associated with large nematode populations. Parthenogenesis did not occur. Ten fertile hybrids between races were produced and the host range of five of these was tested. On average the hybrids multiplied less than their parent races and their host ranges showed no relationship to those of their parent races. RCR and TR inoculated together into tulip produced significantly fewer nematodes than did TR alone and more than RCR alone. Mixed populations of races occur in nature which are probably a mixture of parental races, their hybrids and back-crosses. Repeated back-crossing of the hybrids with parental types and the slower multiplication of the hybrids is an explanation for the variation in host range of known races and the failure to record new races.     

EXTREME KARYOTYPIC VARIATION IN A MUS MUSCULUS DOMESTICUS HYBRID ZONE: THE TOBACCO MOUSE STORY REVISITED

The Robertsonian fusion is a common chromosomal mutation among mammal species and is especially prevalent in the West European house mouse, Mus musculus domesticus. More than 40 races of the house mouse exist in Europe, including the famous “tobacco mouse” (Poschiavo race) of Val Poschiavo, Switzerland. Documented here is the discovery of an extreme case of karyotypic variation in the neighboring Upper Valtellina, Italy. In a 20-km stretch of the valley, 32 karyotypes were observed, including five chromosomal races and 27 hybrid types. One previously unknown race is reported, the “Mid Valtellina” race, with a diploid number of 2n = 24 and the Robertsonian fusions Rb(1.3), Rb(4.6), Rb(5.15), Rb(7.18), Rb(8.12), Rb(9.14), Rb(11.13), and Rb(16.17). The Poschiavo race (2n = 26), Upper Valtellina race (2n = 24), Lower Valtellina race (2n = 22) and all-acrocentric race (2n = 40) were also present. The races form a patchy distribution, which we term a “mottled hybrid zone.” Geographical position, isolation, extinction, recolonization, and selection against hybrids are all believed to be instrumental in the origin and evolution of this complex system. Previous studies of house mice from Upper Valtellina indicated that two of the races in the valley (the Upper Valtellina and Poschiavo races) may have speciated in the village of Migiondo. We discuss the possibility that there may have been a reinforcement event in this village.     

The cytology of Brachycome lineariloba

A comparison of the karyotypes of races D (2n=8), E (2n=10), B (2n=12) and C (2n=16) of B. lineariloba suggests that these races have in common a basic set of four chromosome pairs, and that the higher chromosome number races are related to race D by successive chromosome addition. — A study of meiosis in B × C and A₁ × B hybrids supports this contention and elucidates the homologies of the additional chromosomes. — Meiotic pairing in hybrids between A and C is very complex. At present it can only be stated that there are extensive interchromosomal homologies between the two races. — Two phyletic schemes of the relationships of the races are considered. The second, which is favoured, involves successive chromosome addition, with the quasidiploid race E (2n = 10) giving rise to race B by diploidisation of the univalent chromosomes. This scheme is supported by features of univalent behaviour in the various races and their hybrids. — The ecogeographic distribution pattern of the races shows replacement of D by E by B by C as the species extends into more arid and more harsh environments. This replacement is also associated with increasing vigour. — It seems most likely that the addition chromosomes are derived from a race A (2n=4) source since they are added always by twos, and each addition increases both vigour and drought tolerance. Race A is the most vigorous and one of the most drought tolerant of the five races.It is suggested that the evolution of the races can be related to the increasing aridity of the Late Pleistocene and Recent geological epochs.     

Ophiostoma novo-ulmi sp. nov., causative agent of current Dutch elm disease pandemics

The aggressive subgroup of the Dutch elm disease pathogen Ophiostoma ulmi (Buism.) Nannf. syn. Ceratocystis ulmi (Buism.) Moreau is named as a new species, O. novo-ulmi, and is thereby separated from the old non-aggressive subgroup, which is retained as O. ulmi. O. novo-ulmi differs from O. ulmi in colony morphology, growth rate, optimum temperature for growth, perithecial neck length, pathogenicity to elm, bark colonising ability, cerato-ulmin protein production, synnemetal and protoperithecial production, mating type frequency, protein and isozyme polymorphisms, mitochondrial DNA and nuclear DNA polymorphisms, and mitochondrial DNA size. In addition, a strong unidirectional fertility barrier operates between the two species, while their hybrids show remarkable variation, poor fitness, and many are infertile. These aspects are summarised. New information on perithecial dimensions is presented. O. ulmi is redefined and a neotype designated. The status of the Eurasian and North American races of O. novo-ulmi is currently under investigation.Abbreviations EAN Eurasian race - NAN North American race     

GENETIC DIFFERENTIATION OF POLISH POPULATIONS OF SOREX ARANEUS L. III. INTERCHROMOSOMAL RECOMBINATION IN A HYBRID ZONE

Two parapatric chromosomal races of the common shrew (Sorex araneus) in Poland differ in their complement of metacentric arm combinations: hk, io, gr, nm (race IV), and hi, ko, gm, np (race II). In hybrids, these eight race-diagnostic metacentrics form two randomly segregating complexes. The first complex (C₁) occurs in the form of a ring configuration ok/kh/hi/io, or a chain o/ok/kh/hi/i (when there is Robertsonian polymorphism of the element io). The second complex (C₂) always takes the form of a six-element chain configuration r/rg/gm/mn/np/p. The C₂ complex may be shortened to five or even four elements, when acrocentrics g, m and n are present. In the contact zone we found shrews of pure races (race II or IV), as well as hybrids with C₁ or C₂ complexes, and recombinants hi, ko, gr, nm. Complex heterozygotes are likely to suffer reduced fertility due to malsegregation at meiosis. However, the C₁ hybrids with ring configurations occur with a high frequency throughout the contact zone. This suggest that their fitness is only slightly lowered relative to pure race individuals, in contrast to the hybrids with C₁ or C₂ chain configurations, which presumably have a more heavily reduced fertility. On the other hand, at the center of the zone there is a high proportion of recombinants, which, being chromosomal homozygotes, should display normal meiotic segregation. Furthermore, the high frequencies of recombinants within the contact zone should facilitate gene flow between the races. The occurrence of recombinants plays a similar role as the appearance of the maximum frequencies of acrocentric homozygotes described in several contact zones of S. araneus.     

An electron microscopic study of the etiology of hybrid sterility in Drosophila paulistorum

Crosses between the semispecies of the Drosophila paulistorum complex produce fertile female but sterile male hybrids. An hypothesis is put forward that the hybrid sterility is in this case a result of discordance between a cytoplasmic symbiont and the genotype of the host. An ultrastructural analysis has been made of the testes of sterile male hybrids between the Andean and the Amazonian, and also other semispecies. Spermatid bundles undergo degenerative changes resulting in the loss of the axial filament complex and of associated mitochondrial derivatives. Numerous single or multiply clustered elements closely resembling Mycoplasma are observed in association with the degenerating spermatid bundles. Similar inclusions are observed also intruding into the wall of the distal parts of the testes and/or vasa deferentia. Some Mycoplasma-like bodies are observed also within the developing spermatids. These organs may be crowded with degenerating bundles and clusters of the Mycoplasma-like bodies. Each body is enclosed in a membrane, and may show a central reticular network and peripheral ribosome-like granules. The testes of the fertile males of the parental stocks reveal the presence of similar Mycoplasma-like inclusions, but not in such profusion.Dedicated to Professor Theodosius Dobzhansky on the occasion of his seventieth birthday, and in gratitude for our introduction to Drosophila paulistorum.     

STRUCTURE AND CHEMISTRY OF A NEW CHEMICAL RACE OF BOTRYOCOCCUS BRAUNII (CHLOROPHYCEAE) THAT PRODUCES LYCOPADIENE,A TETRATERPENOID HYDROCARBON1

New strains of the hydrocarbon rich alga Botryococcus braunii Kützing were isolated from water samples collected in three tropical freshwater lakes. These strains synthesize lycopadiene, a tetraterpenoid metabolite, as their sole hydrocarbon. The morphological and ultrastructural characteristics of these algae are similar to those reported for previously described strains which produce either alkadienes or botryococcenes. The pyriform shaped cells are embedded in a colonial matrix formed by layers of closely appressed external walls: this dense matrix is impregnated by the hydrocarbon and some other lipids. We believe the new strains synthesizing lycopadiene form a third chemical race in B. braunii, besides the alkadiene and botryococcene races, rather than a different species. Like the other two types of hydrocarbons, lycopadiene was produced primarily during the exponential and linear growth phases. The major fatty acid in the three races was oleic acid. This fatty acid was predominant in the alkadiene race; palmitic and octacosenoic acid also were present in appreciable amounts in the three races. Cholest-5-en-3β-ol, 24-methylcholest-5-en-3β-ol and 24-ethylcho-lest-5-en-3β-ol occurred in the three races; three unidentified sterols also were detected in the lycopadiene race. Moreover, the presence of very long chain alkenyl-phenols in the lipids of algae of the alkadiene race was not observed in the botryococcene and lycopadiene races. Of the polysaccharides released in the medium, galactose appeared as a primary component: it predominated in the botryococcene race. The other major constituents were fucose for the alkadiene race and glucose and fucose for the lycopadiene race. Although morphologically similar, some important chemical differences exist among algae classified as B. braunii.     

Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

The inheritance of restriction fragment length polymorphisms in the flax rust Melampsora lini

Summary Random cDNA sequences synthesized from poly A⁺ RNA extracted from germinated urediospores of the flax rust fungus, Melampsora lini, were used as probes to detect restriction fragment length polymorphisms (RFLPs) in three races of M. lini originating from cultivated flax, Linum usitatissimum, and one race originating from Australian native flax, L. marginale. Fourteen out of 22 probes tested detected RFLPs in the three races from cultivated flax while 19 of the probes detected polymorphisms between these three races and the race from L. marginale. The segregation of seven RFLPs was determined in a family of 19 F₂ progeny derived from a cross between two of the rust races. With six of these the inheritance was consistent, in each case, with the segregation of alleles at a single locus. Inheritance of the seventh was unusual and an explanation involving two loci with null alleles at each was proposed. No linkage was detected between any of the RFLP loci and nine unlinked loci specifying avirulence.     

FLAVONOID RACES OF CLAYTONIA VIRGINICA (PORTULACACEAE)

Four flavonoid races have been found in the Claytonia virginica aneuploid complex, differing from one another in the accumulation of 12 kaempferol and quercetin 3-glycosides. Each of these races has a distinctive geographical distribution within the overall range of the species. The three major diploid cytotypes of C. virginica each belong to different races, two of which also include polyploids; a fourth race consists entirely of polyploids. An examination of biosynthetic pathways indicates that a small number of genetic changes are responsible for the observed variation; polyploidy per se does not appear to have contributed significantly to the production of novel compounds. Flavonoid data also suggest strongly that polyploidy within rather than between modern chemical races has been responsible for the bulk of chromosome number variation in the species.     

Genetic variation and differentiation of wood mice from the genus <Emphasis Type="Italic">Sylvaemus</Emphasis> inferred from sequencing of the cytochrome oxidase subunit 1 gene fragment

To ascertain intra- and interspecific differentiation patterns of some Sylvaemus wood mice species (S. uralensis, S. sylvaticus, S. ponticus, S. flavicollis, and S. fulvipectus), sequence variation of the fragment (654 bp) of the mitochondrial cytochrome oxidase subunit I gene (COI) was analyzed and the data obtained using several molecular genetic markers were compared. Distinct isolation of all Sylvaemus species (including closely related allopatric S. flavicollis and S. ponticus), as well as of the European and Asian races of pygmy wood mouse S. uralensis at the COI gene was demonstrated. However, genetic differences of the Sylvaemus species were 1.5 times and more higher than the distance (D) between the races of S. uralensis. This finding provides no ample grounds to treat them as the independent species. The only examined specimen of Pamir-Alay subspecies S. uralensis pallipes showed closest relatedness to to the Asian race, although was rather distant from it (D = 0.038). No reliable isolation of the eastern European and southern European chromosomal forms, representing the European race of S. uralensis, as well as of their presumptive hybrids from the out-skirts of the city of Sal’sk, Rostov region, at the COI gene was revealed. A hybrid origin of the population of pygmy wood mouse from the outskirts of the Talapker railway station, Novovarshavsky district, Omsk region, was confirmed (in preliminary studies, based on karyotypic characters, these specimens were diagnosed as distant hybrids of the eastern European chromosomal form and the Asian race). In yellow-necked wood mouse S. flavicollis from the territory of Russia and Ukraine, weak differentiation into northern and southern lineages (with mean genetic distance between them of 0.020) was observed. Considerably different relative genetic distances between the races of S. uralensis and the S. flavicollis-S. ponticus species pair, inferred from the mitochondrial cytochrome oxidase and cytochrome b genes data, indicated that the rates of evolution of the mitochondrial genome regions could be very dissimilar. It is suggested that transformations of the cytochrome b gene, or at least its part, were irregular in time and/or in different phyletic lineages (i.e., accelerated upon the formation of pygmy wood mouse races, and delayed upon the establishment of S. flavicollis and S. ponticus).     

Determination of Physiological Races and Evaluation of Sunflower for Resistance to Puccinia helianthi Schw

Sunflower rust, caused by Puccinia helianthi Schw., is a widespread disease of sunflower (Helianthus annuus L.) in China. To study physiological races, sunflower field surveys were undertaken in major sunflower growing areas of China in 2010. Forty‐four rust‐infected sunflower leaf samples were collected from 25 geographical locations. Freshly produced spores were used to study physiological race differentiation on a set of nine differentials. Race 300 was the most prevalent race observed over all locations with a 59% frequency followed by races 735, 310, 500, 724 and 737. To evaluate hybrids and varieties for resistance screening, spores of race 300 were used to inoculate 65 hybrids, and five open‐pollinated varieties selected from breeding programmes and from the seed market. None of the confection hybrids and open‐pollinated varieties was immune to race 300. Conversely, among oilseed hybrids, 3% of them showed immunity, 12% highly resistant, 59% resistant and 26% showed susceptible reactions. Open‐pollinated varieties were the most susceptible to race 300 followed by confection and oilseed sunflower hybrids. Results from this study are projected to assist breeders in selection of hybrids and varieties against prevalent race as our results showed a diversity of resistance levels to race 300.     

Geographical variation of chromosomal structure in Drosophila gasici

Summary Drosophila gasici Brncic 1957, is a neotropical species found in several parts of the Andes Mountain System. By means of the analysis of their external characteristics, chromosomes and hybridization test they have been included in the mesophragmatica group of species of the sub-genus Drosophila.The present paper describes the mitotic and polytene chromosomes of D. gasici from samples of natural populations collected at Bogotá (Colombia), Cochabamba (Bolivia), Arica (Chile) and San Luis (Argentina). The comparative study of all these populations has disclosed that the species has split in well defined geographic races. The Colombian and Chilean flies differ from those living in Bolivia and Argentina by three independent inversions in chromosome I (the sexual pair). The only polymorphic populations seem to be the Chilean ones which exhibit two inversions in the second chromosome, besides the Standard gene arrangement. All the other populations are homozygous for all their chromosomal sequences. Studies on reproductive isolation have demonstrated that there is some sexual discrimination between the Colombian and Chilean flies in respect to the Bolivian and Argentinean ones.The populational structure of D. gasici is in contrast to that observed in the other six species belonging to the mesophragmatica group in which there are no good evidences of geographical variations at the chromosomal level.This article is dedicated to Professor Hans Bauer on the occasion of his 60th birthday.     

New Races of the Sunflower Downy Mildew Pathogen (Plasmopara halstedii) in Europe and North and South America

Collections of the causal agent of sunflower downy mildew (Plasmopara balstedii) (PH) from several European and North and South American countries were used to inoculate a series of sunflower inbred lines to differentiate races. Race 3 was identified from Argentina; race 4 from France, Hungary, and Bulgaria; race 6 from Canada and France; and race 7 from Argentina. This is the first report of race 3 in South America and of race 4 in Europe, and is the first identification of races 6 and 7. Reactions of USDA lines RHA-274, RHA-325, and DM-2 differentiate races 2, 6, and 7, with RHA-274 resistant to all three. Thirty-three commercial hybrids from 11 countries were tested; 70 % were resistant to race 2, only 27 % had resistance to race 6, and none were resistant to race 4. Surveys to determine the relative proportions of the various races within each country are needed to assess their potential impact. The USDA lines HA-335, HA-336, HA-337, HA-338, HA-339, RHA-340, HA-R4, and HA-R5 are resistant to all seven mildew races and could be utilized for the production of PH resistant hybrids. For the short term, it appears that seed treatment with metalaxyl may be the most effective control.     

A second case of genetic host races in Rhagoletis? A population genetic comparison of sympatric host populations in the European cherry fruit fly,Rhagoletis cerasi

Despite an increasing acceptance in the biological community for sympatric speciation as a mode of species formation, well documented examples of sympatrically evolved ‘incipient species’ remain rare. The sympatric host races of apple maggot, Rhagoletis pomonella (Walsh), represent one of the most prominent case studies for sympatric speciation via a host shift. The European cherry fruit fly, R. cerasi (L.), shows strong ecological similarities to R. pomonella: (1) infestation of two different host plants, Lonicera xylosteum L. and Prunus avium L., and (2) divergent phenological and behavioral adaptations of flies on different hosts. The population genetic study presented here addresses whether the host associated populations of R. cerasi also represent genetically differentiated true host races. Out of a total of 29 allozyme loci examined, six were polymorphic and used to analyze six sympatric pairs of R. cerasi populations on Lonicera and Prunus from Switzerland and Germany. A direct comparison of allele frequencies between sympatric sites showed no pattern indicative of host races in R. cerasi. However, the hierarchical F‐statistic for one locus, mannose 6‐phosphate isomerase (Mpi), showed significant population differentiation that was in accordance with host race differentiation. Mpi is one of several loci that are also diagnostic for host race differentiation in R. pomonella. Results from Mpi suggest the formation of sympatric host races in R. cerasi, but additional polymorphic markers are necessary.