Tissue elastic properties of eight Hawaiian Dubautia species that differ in habitat and diploid chromosome number

Summary Eight Hawaiian Dubautia species grow in habitats as varied as exposed lava, dry scrub, mesic forest, wet forest, and bog. These species also differ in diploid chromosome number, with four species having 13 pairs of chromosomes and four species having 14 pairs. This ecological and chromosomal variation is paralleled by significant interspecific variation in tissue elastic properties. The four 13-paired species from dry habitats exhibit significantly lower tissue elastic moduli near full hydration (E _i) than the four 14-paired species from mesic to wet habitats. Values of E _i range from 2 to 4 MPa among the former species and from 9 to 18 MPa among the latter species. The turgor dependence of the elastic modulus also differs markedly between the two groups of species. As a result of these differences in tissue elastic properties, the capacity for maintaining high turgor pressures as tissue water content decreases is much greater in the 13-paired species from dry habitats than in the 14-paired species from mesic to wet habitats. These results indicate that the evolutionary diversification of the Dubautia species has been accompanied by a significant degree of change at the physiological level.     

POLYPLOIDY,DYSPLOIDY, AND CHROMOSOME PAIRING IN ECHEVERIA (CRASSULACEAE) AND ITS HYBRIDS

The 140+ species of Echeveria have more than 50 gametic chromosome numbers, including every number from 12 through 34 and polyploids to n = ca. 260. With related genera, they comprise an immense comparium of 200+ species that have been interconnected in cultivation by hybrids. Some species with as many as 34 gametic chromosomes include none that can pair with each other, indicating that they are effectively diploid, but other species with fewer chromosomes test as tetraploids. Most diploid hybrids form multivalents, indicating that many translocations have rearranged segments of the chromosomes. Small, nonessential chromosomal remnants can be lost, lowering the number and suggesting that higher diploid numbers (n = 30–34) in the long dysploid series are older. These same numbers are basic to most other genera in the comparium (Pachyphytum, Graptopetalum, Sedum section Pachysedum), and many diploid intergeneric hybrids show very substantial chromosome pairing. Most polyploid hybrids here are fertile, even where the parents belong to different genera and have very different chromosome numbers. This seems possible only if corresponding chromosomes from a polyploid parent pair with each other preferentially, strong evidence for autopolyploidy. High diploid numbers here may represent old polyploids that have become diploidized by loss, mutation, or suppression of duplicate genes, but other evidence for this is lacking. Most species occur as small populations in unstable habitats in an area with a history of many rapid climatic and geological changes, presenting a model for rapid evolution.     

KARYOTYPES OF IRIS PUMILA AND RELATED SPECIES

Randolph , L. F. and Jyotirmay Mitra . (Cornell U., Ithaca.) Karyotypes of Iris pumila and related species. Amer. Jour. Bot. 46(2): 93-102. Illus. 1959.—The karyotypes of 30- and 32-chromosome geographical variants of the amphidiploid I. pumila from Russia and the Balkans were compared with the karyotype of the typical 32-chromosome Austrian forms of this species and with those of the diploid I. attica and I. pseudopumila, previously reported to be the basic species from which I. pumila originated. Plants from 3 collections of a Crimean form of I. pumila with 32 chromosomes had a pair of long chromosomes with submedian centromeres morphologically similar to chromosome 1 of the typical form of I. pumila. In addition, there was another heteromorphic pair of submedian chromosomes with one of the members having a shorter short arm. The manner in which this altered chromosome could have arisen as a result of a heterobrachial inversion is described. Five different collections of I. pumila with 30 chromosomes from Russia differ in several respects from the typical 32-chromosome I. pumila. They have an unusually long pair of chromosomes with a submedian centromere and a secondary constriction in the long arm. This chromosome is the original chromosome 2 which had been altered by the addition of a segment equivalent to the most of the long arm of one of the shorter chromosomes with subterminal centromere. The manner in which this could have occurred as the result of unequal reciprocal translocation is described. Loss of the remaining diminutive portion of the short chromosome with subterminal constriction assumed to have been involved in the unequal interchange of segments producing the modified, longer chromosome 2 would account for the reduction in chromosome number from 32 to 30 in the Russian form of I. pumila. Four pairs of chromosomes with satellites have been found in the 30-chromosome plants whereas 6 pairs of satellited chromosomes are present in the 32-chromosome I. pumila. The spontaneous occurrence of chromosomal alterations of the type here described are considered to be significant factors in the process of chromosomal repatterning resulting in the appearance of new geographical races, and eventually of species of iris, with altered chromosome numbers and modified karyotypes. More specifically it is concluded that amphidiploidy accompanied by chromosomal repatterning resulting from segmental interchange, heterobrachial inversion and related types of chromosomal alterations has played an important role in the evolution of I. pumila and karyological forms of this species occupying different geographical areas.     

CHROMOSOME NUMBERS OF HAWAIIAN FLOWERING PLANTS AND THE SIGNIFICANCE OF CYTOLOGY IN SELECTED TAXA

Chromosome numbers are reported for 128 species of flowering plants indigenous or endemic to Hawaii, including first reports for 13 genera and 82 species. The special significance of reports for Ilex, Tribulus, Keysseria, Pisonia, Boerhavia, Jacquemontia, Claoxylon, Lipochaeta, Railliardia, and Dubautia are discussed. The cytological and morphological variation in Railliardia and Dubautia is considered and their treatment as congeners is advocated. The cytogeogruphic pattern in Dubautia and Railliardia and other factors suggest that the ancestral chromosome number of the Hawaiian tarweeds is n = 14. Their derivation from a western North American progenitor similar to Adenothamnus is considered plausible if not indeed likely.     

CYTOGENETICS OF HYBRIDS OF CARTHAMUS SPECIES (COMPOSITAE) WITH TEN PAIRS OF CHROMOSOMES

Plants thought to be typical of 7 species were chosen to represent the various taxa of Carthamus species with 10 pairs of chromosomes. These entities were crossed in all possible combinations and 20 of the possible 21 interspecific hybrids were obtained after 3 seasons of crosses. Analyses of the hybrids included studies of microsporocytes, pollen stainability, achene fertility, rudimentary ovaries, and other morphological characteristics. Pairing of chromosomes at metaphase I indicated no translocations were present in hybrids between C. tenuis from Israel, C. alexandrinus from Egypt, C. glaucus from northern Israel, and C. syriacus from Jordan. Members of this group are assigned the standard arrangement. Hybrids of C. glaucus from Iran, C. glaucus from Syria, and C. dentatus from Turkey always showed a translocation or chromosomal interchange when crossed with any member having the “standard” arrangement. The last 3 species are considered to have the “non-standard” chromosomal arrangement. The parental species used in this study can be regarded as a set of testers which will allow identification of chromosomal differentiation in additional Carthamus materials as they are collected.     

Empirical demonstration of hybrid chromosomal races in house mice

Western house mice (Mus musculus domesticus) and common shrews (Sorex araneus) are important models for study of chromosomal speciation. Both had ancestral karyotypes consisting of telocentric chromosomes, and each is subdivided into numerous chromosomal races many of which have resulted from fixation of new mutations (Robertsonian fusions and whole‐arm reciprocal translocations). However, some chromosomal races in both species may alternatively have originated through hybridization, with particular homozygous recombinant products reaching fixation. Here, we demonstrate the process of generation of hybrid chromosomal races for the first time in either species using molecular markers. Analysis of centromeric microsatellite markers show that the Mid Valtellina (IMVA) and Upper Valtellina (IUVA) chromosomal races of the house mouse are recombinant products of hybridization of the Lower Valtellina (ILVA) and Poschiavo (CHPO) chromosomal races, supporting earlier theoretical analysis. IMVA and IUVA occupy a small area of the Italian Alps where ILVA makes contact with CHPO. IUVA and CHPO have previously been shown to be reproductively isolated in one village, emphasizing that hybrid chromosomal races in small mammals, as in plants, have the potential to be part of the speciation process.     

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.     

Cytogenetic studies in the speciesCercopithecus pogonias (Bennet 1833) andCercopithecus nictitans (Linnaeus 1966)

We describe the banding patterns of the chromosomes of Cercopithecus pogonias(2n = 72) and Cercopithecus nictitans nictitans(2n = 70), the two species which exhibit the highest diploid numbers among the Cercopithecidae, using G-banding, C-banding, and nucleolar organizing region (NOR)-staining techniques. The karyotypes of these two species show a large number of morphological homologies, but several chromosome pairs cannot be matched. It is suggested that translocations and insertions may have been important in the chromosomal evolution of this group.     

The role of chromosome rearrangements in the evolution of mole voles of the genus Ellobius (Rodentia,Mammalia)

Modern mole voles of the genus Ellobius are characterized by species-specific features of autosomes and sex chromosomes. Owing to the use of the Zoo-FISH method, the nomenclature of chromosomes was refined and nonhomologous Robertsonian translocations indistinguishable by G-staining were identified for Ellobius tancrei, which is a species with a wide chromosome variation of the Robertsonian type. The electron-microscopic analysis of synaptonemal complexes in F₁ hybrids of forms with 2n = 50 and 2n = 48 revealed the formation of a closed SC-pentavalent composed of three metacentrics with monobrachial homology and two acrocentrics. Segregation of chromosomes of such complex systems is impeded by disturbances in the nucleus architecture leading to the formation of unbalanced gametes and to a dramatic reduction in fertility of hybrids. Our data support the hypothesis that the formation of monobrachial homologous metacentric chromosomes can be considered as a way of chromosomal speciation.     

Cytogenetic Analysis of Experimental Hybrids in Species of Triatominae (Hemiptera-Reduviidae)

A cytogenetic analysis was performed in experimental hybrids between species of Chagas disease transmitting bugs with remarkable differences in the amount and distribution of heterochromatin. Using C-banding technique, we identified the parental species chromosomes and analysed the meiotic behaviour in the male hybrids between Triatoma platensis and T. infestans, T. platensis and T. delpontei, and T. infestans and T. rubrovaria. The two former hybrids have an entirely normal meiotic behaviour despite the extensive differences in C-banded karyotypes observed in the parental species, indicating that heterochromatin differences between homeologous chromosomes are not a barrier that influences meiotic synapsis and recombination. On the contrary, the experimental hybrids between T. infestans and T. rubrovaria show failures in pairing of homeologous chromosomes that lead to the production of abnormal spermatids and hybrid sterility. Our data suggest that karyotypic repatterning within triatomines has involved at least two different pathways. Among closely related species, chromosomal changes have largely involved addition or deletion of heterochromatic regions. In more distant species, chromosomal rearrangements (i.e. inversions and translocations) have also arisen. Hybridisation data also allow to hypothesize about the origin and divergence of this taxonomic group, as well as the mechanisms that maintain species isolation.     

CHROMOSOME RACES AND STRUCTURAL HETEROZYGOSITY IN CALYCADENIA CILIOSA GREENE (ASTERACEAE)

A biosystematic study of Calycadenia ciliosa resulted in the recognition of five homoploid (n = 6) chromosome races. The cytogenetic evidence indicates that each of these races is differentiated from its nearest relative by a single reciprocal chromosome translocation, although at one point in the evolutionary history a pericentric inversion may have been a concurrent cytological event. The data also show that the chromosome phylogeny includes two instances of redundant translocation. Mean pollen stainabilities of interracial hybrids range from 16–80%. In a survey of four natural populations 30–60% of the individuals sampled were found to be structurally heterozygous for reciprocal chromosome translocations. Pericentric inversion heterozygosity was also detected in one population. Another population contained morphologically indistinguishable individuals separated by as little as 120 m that were differentiated by a minimum of four chromosome translocations. These observations were compared with similar instances in other species in an effort to determine their significance in the process of plant evolution.     

Identification of the chromosome complement and the spontaneous 1R/1V translocations in allotetraploid <Emphasis Type="Italic">Secale cereale</Emphasis> × <Emphasis Type="Italic">Dasypyrum villosum</Emphasis> hybrids through cytogenetic approaches

Genome modifications that occur at the initial interspecific hybridization event are dynamic and can be consolidated during the process of stabilization in successive generations of allopolyploids. This study identifies the number and chromosomal location of ribosomal DNA (rDNA) sites between Secale cereale, Dasypyrum villosum, and their allotetraploid S. cereale × D. villosum hybrids. For the first time, we show the advantages of FISH to reveal chromosome rearrangements in the tetraploid Secale × Dasypyrum hybrids. Based on the specific hybridization patterns of ribosomal 5S, 35S DNA and rye species-specific pSc200 DNA probes, a set of genotypes with numerous Secale/Dasypyrum translocations of 1R/1V chromosomes were identified in successive generations of allotetraploid S. cereale × D. villosum hybrids. In addition we analyse rye chromosome pairs using FISH with chromosome-specific DNA sequences on S. cereale × D. villosum hybrids.     

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.     

Genetics of speciation in the Aedes (Stegomyia) scutellaris group (Diptera: Culicidae) V. Chromosomal relationships among five species

A comparison was made of karyotypes of 5 species in the Aedes scutellaris group and their hybrids. All species had 3 distinct pairs of metacentric chromosomes (2n=6). These were of similar lengths in Ae: malayensis and Ae. alcasidi, and in Ae. polynesiensis and Ae. pseudoscutellaris. However, chromosome 1 in Ae. polynesiensis and Ae. kesseli, and chromosome 2 in Ae. pseudoscutellaris and Ae. kesseli were of unequal lengths. Meiotic analyses revealed that chromosome asynapsis was frequently seen in species hybrids. There was a significant variation in chiasma frequencies between species and their hybrids. However, the mean chiasma frequency was species specific. In addition, the mean chiasma frequency of species hybrids and the extent of chromosomal asynapsis provided a measure of genetic homology between species. Based on the assumption that a dicentric bridge and an acentric fragment were due to a single crossover within a paracentric inversion loop the following conclusions are made. Ae. malayensis and Ae. alcasidi are polymorphic for one paracentric inversion in chromosome 1. Ae. polynesiensis and Ae. pseudoscutellaris, and Ae. pseudoscutellaris and Ae. kesseli are fixed for one paracentric inversion in chromosome 2. Similarly, Ae. polynesiensis and Ae. kesseli are fixed for one paracentric inversion in chromosome 1. These chromosomal differences between species are discussed with respect to hybrid fertility data reported earlier.     

Phyletic and evolutionary relationships ofBrachyscome lineariloba (Compositae)

A comparison of karyotypes ofBrachyscome breviscapis (2n = 8),B. lineariloba cytodemes E (2n = 10), B (2n = 12) and C (2n = 16) suggests that these species have a homoelogous basic set of four chromosome pairs, two large pairs and two small, and that theB. lineariloba cytodemes E, B and C are related toB. breviscapis by successive additions of small chromosomes. A pronounced asynchrony of chromosome condensation between these large and small chromosomes has been observed. In the artificial hybrids betweenB. dichromosomatica (2n = 4) ×B. breviscapis, and theB. lineariloba cytodemes, theB. dichromosomatica chromosomes are similar in size and condensation behaviour to the small chromosomes ofB. breviscapis and ofB. lineariloba cytodemes E, B and C. Meiotic pairing in these hybrids also demonstrates the strong affinities between these chromosomes. It is suggested thatB. breviscapis may be of amphidiploid origin between a species with two large early condensing chromosome pairs and another,B. dichromosomatica-like species with two small late condensing pairs. It seems most likely that the additional small and late condensing chromosomes inB. lineariloba cytodemes E, B and C are derived from theB. dichromosomatica-like parent, and that each addition increases vigour, fecundity and drought tolerance, allowing these cytodemes to colonize more open and arid environments. Transmission of the univalents in the quasidiploidB. lineariloba cytodeme E was verified as being via the pollen, and not via the embryo sacs.The cytology ofBrachyscome lineariloba (Compositae, Asteroidae), 10.     

CHROMOSOMAL DIFFERENTIATION IN CLARKIA DUDLEYANA

Snow , Richard . (U. California, Davis.) Chromosomal differentiation in Clarkia dudleyana. Amer. Jour. Bot. 47 (4) : 302—309. Illus. 1960.—Clarkia dudleyana (n=9) is a common, colonial annual of the early-summer California flora. Of 275 individuals, derived from 9 natural populations and their garden-grown representatives, 17.1% were heterozygous for reciprocal translocations. Supernumerary chromosomes were also found in about 2% of the plants examined. The translocation heterozygotes are not distributed regularly over the species range but are concentrated near the geographical center of distribution. Most of the populations contained none or only a few heterozygotes, but in one colony 69% of 42 plants sampled were heterozygous. Judging from the meiotic metaphase associations observed, at least 5 different chromosome arrangements are present at this locality. Hybrids between colonies have invariably been translocation heterozygotes, the largest association found in such hybrids being a chain of all 18 chromosomes (a potential ring of 18). No correlation is evident between geographical separation and degree of cytological differentiation. Heterozygotes with smaller rings of 4 or 6 chromosomes, whether from natural populations or resulting from interpopulation hybridization, are highly fertile owing to the regular alternate disjunction of the chromosomes of the rings. In the larger rings of 12 to 18 chromosomes, derived from interpopulation crosses, segregation is much more irregular and leads to high sterility. It is possible that at least in some localities the heterozygotes enjoy a selective advantage over their homozygous sibs. It is also postulated that homozygosity for a particular chromosome arrangement may be selectively favored in a certain habitat, as a result of a position effect attendant upon placing formerly non-linked genes in the same linkage group through reciprocal translocation. The high degree of chromosomal differentiation between some populations of this species suggests that the complex heterozygotes of Oenothera have arisen as a result of hybridization of cytologically differentiated races.     

The chromosomes of two Drosophila races: D. nasuta nasuta and D. n. albomicana

Heterochromatin distribution and differentiation in metaphase chromosomes of two morphologically identical Drosophila races, D. nasuta nasuta and D. n. albomicana, have been studied by C- and N-banding methods. — The total heterochromatin values differ only slightly between these races. However, homologous chromosomes of the two Drosophila forms show striking differences in the size of heterochromatin regions and there is an alternating pattern in D. n. nasuta and D. n. albomicana of chromosomes which contain more, or respectively less heterochromatin than their counterparts in the other race. — Three different N-banding patterns could be obtained depending on the conditions of the method employed: One banding pattern occurs which corresponds to the C-banding pattern. Another pattern is the reverse of the C-band pattern; the euchromatic chromosome regions and the centromeres are stained whereas the pericentric heterochromatin regions remain unstained. In the Y chromosomes of both races and in chromosome 4 of D. n. albomicana, however, the heterochromatin is further differentiated. In the third N-banding pattern only the centromeres are deeply stained. Furthermore, between the races, subtle staining differences in the pericentric heterochromatin regions can be observed as verified in F₁ hybrids. On the basis of C- and N-banding results specific aspects of chromosomal differences between D. n. nasuta and D. n. albomicana are discussed.Dedicated to Prof. W. Beermann on the occasion of his 60th birthday     

Multiple Ty-mediated chromosomal translocations lead to karyotype changes in a wine strain of Saccharomyces cerevisiae.

Enological strains of Saccharomyces cerevisiae display a high level of chromosome length polymorphism, but the molecular basis of this phenomenon has not yet been clearly defined. In order to gain further insight into the molecular mechanisms responsible for the karyotypic variability, we examined the chromosomal constitution of a strain known to possess aberrant chromosomes. Our data revealed that the strain carries four rearranged chromosomes resulting from two reciprocal translocations between chromosomes III and I, and chromosomes III and VII. The sizes of the chromosomal fragments exchanged through translocation range from 40 to 150 kb. Characterization of the breakpoints indicated that the translocations involved the RAHS of chromosome III, a transposition hot-spot on the right arm of chromosome I and a region on the left arm of chromosome VII. An analysis of the junctions showed that in all cases Ty elements were present and suggested that the translocations result from recombination between transposable Ty elements. The evidence for multiple translocations mediated by Ty elements in a single strain suggests that spontaneous Ty-driven rearrangement could be quite common and may play a major role in the alteration of karyotypes in natural and industrial yeasts. Received: 18 December 1998 / Accepted: 26 March 1999     

Fissions,fusions, and translocations shaped the karyotype and multiple sex chromosome constitution of the northeast‐Asian wood white butterfly,Leptidea amurensis

Previous studies have shown a dynamic karyotype evolution and the presence of complex sex chromosome systems in three cryptic Leptidea species from the Western Palearctic. To further explore the chromosomal particularities of Leptidea butterflies, we examined the karyotype of an Eastern Palearctic species, Leptidea amurensis. We found a high number of chromosomes that differed between the sexes and slightly varied in females (i.e. 2n = 118–119 in females and 2n = 122 in males). The analysis of female meiotic chromosomes revealed multiple sex chromosomes with three W and six Z chromosomes. The curious sex chromosome constitution [i.e. W_1–3/Z_1–6 (females) and Z_1–6/Z_1–6 (males)] and the observed heterozygotes for a chromosomal fusion are together responsible for the sex‐specific and intraspecific variability in chromosome numbers. However, in contrast to the Western Palearctic Leptidea species, the single chromosomal fusion and static distribution of cytogenetic markers (18S rDNA and H3 histone genes) suggest that the karyotype of L. amurensis is stable. The data obtained for four Leptidea species suggest that the multiple sex chromosome system, although different among species, is a common feature of the genus Leptidea. Furthermore, inter‐ and intraspecific variations in chromosome numbers and the complex meiotic pairing of these multiple sex chromosomes indicate the role of chromosomal fissions, fusions, and translocations in the karyotype evolution of Leptidea butterflies.     

UNDERSTANDING THE BASIS OF DIMINISHED GENE FLOW BETWEEN HYBRIDIZING CHROMOSOME RACES OF THE HOUSE MOUSE

Speciation may be promoted in hybrid zones if there is an interruption to gene flow between the hybridizing forms. For hybridizing chromosome races of the house mouse in Valtellina (Italy), distinguished by whole‐arm chromosomal rearrangements, previous studies have shown that there is greater interruption to gene flow at the centromeres of chromosomes that differ between the races than at distal regions of the same chromosome or at the centromeres of other chromosomes. Here, by increasing the number of markers along race‐specific chromosomes, we reveal a decay in between‐race genetic differentiation from the centromere to the distal telomere. For the first time, we use simulation models to investigate the possible role of recombination suppression and hybrid breakdown in generating this pattern. We also consider epistasis and selective sweeps as explanations for isolated chromosomal regions away from the centromere showing differentiation between the races. Hybrid breakdown alone is the simplest explanation for the decay in genetic differentiation with distance from the centromere. Robertsonian fusions/whole‐arm reciprocal translocations are common chromosomal rearrangements characterizing both closely related species and races within species, and this fine‐scale empirical analysis suggests that the unfitness associated with these rearrangements in the heterozygous state may contribute to the speciation process.