Collaborative Research Activities of the Arase and Van Allen Probes.

This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017-2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher L -shells up to L ∼ 10 . After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.

Penetration of MeV electrons into the mesosphere accompanying pulsating aurorae.

Pulsating aurorae (PsA) are caused by the intermittent precipitations of magnetospheric electrons (energies of a few keV to a few tens of keV) through wave-particle interactions, thereby depositing most of their energy at altitudes ~ 100 km. However, the maximum energy of precipitated electrons and its impacts on the atmosphere are unknown. Herein, we report unique observations by the European Incoherent Scatter (EISCAT) radar showing electron precipitations ranging from a few hundred keV to a few MeV during a PsA associated with a weak geomagnetic storm. Simultaneously, the Arase spacecraft has observed intense whistler-mode chorus waves at the conjugate location along magnetic field lines. A computer simulation based on the EISCAT observations shows immediate catalytic ozone depletion at the mesospheric altitudes. Since PsA occurs frequently, often in daily basis, and extends its impact over large MLT areas, we anticipate that the PsA possesses a significant forcing to the mesospheric ozone chemistry in high latitudes through high energy electron precipitations. Therefore, the generation of PsA results in the depletion of mesospheric ozone through high-energy electron precipitations caused by whistler-mode chorus waves, which are similar to the well-known effect due to solar energetic protons triggered by solar flares.

Oxygen torus and its coincidence with EMIC wave in the deep inner magnetosphere: Van Allen Probe B and Arase observations.

We investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specific event found on 12 September 2017, using simultaneous observations from the Van Allen Probe B and Arase satellites. It is found that Probe B observed a clear enhancement in the average plasma mass (M) up to 3-4 amu at L = 3.3-3.6 and magnetic local time (MLT) = 9.0 h. In the afternoon sector at MLT ~ 16.0 h, both Probe B and Arase found no clear enhancements in M. This result suggests that the oxygen torus does not extend over all MLT but is skewed toward the dawn. Since a similar result has been reported for another event of the oxygen torus in a previous study, a crescent-shaped torus or a pinched torus centered around dawn may be a general feature of the O+ density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutoff frequency of the EMIC wave, the ion composition of the oxygen torus is estimated to be 80.6% H+, 3.4% He+, and 16.0% O+. According to the linearized dispersion relation for EMIC waves, both He+ and O+ ions inhibit EMIC wave growth and the stabilizing effect is stronger for He+ than O+. Therefore, when the H+ fraction or M is constant, the denser O+ ions are naturally accompanied by the more tenuous He+ ions, resulting in a weaker stabilizing effect (i.e., larger growth rate). From the Probe B observations, we find that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere.

Multiple time-scale beats in aurora: precise orchestration via magnetospheric chorus waves.

The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.

Atomic resolution electron microscopy in a magnetic field free environment.

Atomic-resolution electron microscopes utilize high-power magnetic lenses to produce magnified images of the atomic details of matter. Doing so involves placing samples inside the magnetic objective lens, where magnetic fields of up to a few tesla are always exerted. This can largely alter, or even destroy, the magnetic and physical structures of interest. Here, we describe a newly developed magnetic objective lens system that realizes a magnetic field free environment at the sample position. Combined with a higher-order aberration corrector, we achieve direct, atom-resolved imaging with sub-Å spatial resolution with a residual magnetic field of less than 0.2 mT at the sample position. This capability enables direct atom-resolved imaging of magnetic materials such as silicon steels. Removing the need to subject samples to high magnetic field environments enables a new stage in atomic resolution electron microscopy that realizes direct, atomic-level observation of samples without unwanted high magnetic field effects.

The Space Physics Environment Data Analysis System (SPEDAS).

With the advent of the Heliophysics/Geospace System Observatory (H/GSO), a complement of multi-spacecraft missions and ground-based observatories to study the space environment, data retrieval, analysis, and visualization of space physics data can be daunting. The Space Physics Environment Data Analysis System (SPEDAS), a grass-roots software development platform (www.spedas.org), is now officially supported by NASA Heliophysics as part of its data environment infrastructure. It serves more than a dozen space missions and ground observatories and can integrate the full complement of past and upcoming space physics missions with minimal resources, following clear, simple, and well-proven guidelines. Free, modular and configurable to the needs of individual missions, it works in both command-line (ideal for experienced users) and Graphical User Interface (GUI) mode (reducing the learning curve for first-time users). Both options have "crib-sheets," user-command sequences in ASCII format that can facilitate record-and-repeat actions, especially for complex operations and plotting. Crib-sheets enhance scientific interactions, as users can move rapidly and accurately from exchanges of technical information on data processing to efficient discussions regarding data interpretation and science. SPEDAS can readily query and ingest all International Solar Terrestrial Physics (ISTP)-compatible products from the Space Physics Data Facility (SPDF), enabling access to a vast collection of historic and current mission data. The planned incorporation of Heliophysics Application Programmer's Interface (HAPI) standards will facilitate data ingestion from distributed datasets that adhere to these standards. Although SPEDAS is currently Interactive Data Language (IDL)-based (and interfaces to Java-based tools such as Autoplot), efforts are under-way to expand it further to work with python (first as an interface tool and potentially even receiving an under-the-hood replacement). We review the SPEDAS development history, goals, and current implementation. We explain its "modes of use" with examples geared for users and outline its technical implementation and requirements with software developers in mind. We also describe SPEDAS personnel and software management, interfaces with other organizations, resources and support structure available to the community, and future development plans. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11214-018-0576-4) contains supplementary material, which is available to authorized users.

On the quantitativeness of grain boundary chemistry using STEM EDS: A ZrO2 Σ9 model grain boundary case study.

Atomic-resolution energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) has recently been shown to be a powerful approach to investigate local chemistry of nanoscale structures quantitatively. While most of the studies have been focused on the quantification of the chemical composition in bulk crystals, few were discussed on interfaces. In this study, we theoretically explored the applicability of STEM EDS for the quantification of local chemistry in grain boundaries (GBs), where the electron channeling can be dramatically changed compared with the bulk due to non-periodic atomic arrangement. We find that: (1) line scan analysis across the GBs or mapping analysis, which have been widely used for interface analysis, sometimes leads to misinterpretation of true interface chemistry. (2) Tilting the specimen, which is effective to reduce the effects of scattering, is not always useful for the quantification of GBs. (3) EDS analysis covering the whole GB structure unit, such as using a box scan, can provide true chemical information. Our study provides useful insights into characterization of interface chemistry using STEM EDS.

Pulsating aurora from electron scattering by chorus waves.

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

Comparative cytogenetics of the African elephant (Loxodonta africana) and Asiatic elephant (Elephas maximus).

G- and C-banded karyotypes of the two extant species of the mammalian order Proboscidea are presented for the first time. Chromosome complements were 2n = 56 in both Loxodonta africana and Elephas maximus. Comparisons between the species demonstrated a high level of chromosome band homology, with 26 conserved autosomal pairs. The normal diploid karyotype of L. africana had 25 acrocentric/telocentric and two metacentric/submetacentric autosomal pairs. E. maximus differed by having one less acrocentric and one additional submetacentric pair due to either a heterochromatic arm addition or deletion involving autosomal pair 27. Several acrocentric autosomes of L. africana exhibited small short arms that were absent in homologous chromosomes of E. maximus. The X chromosomes in both species were large submetacentric elements and were homologous. However, the small acrocentric Y chromosomes differed; in E. maximus it was slightly larger and had more distinct G-bands than its counterpart in L. africana. Extant Elephantidae appear to be relatively conservative in their rates of chromosomal change compared to some other mammalian families. The high-quality banded karyotypes presented here should prove useful as references in future chromosome analyses of elephant populations and in comparative cytogenetic studies with other ungulate orders.

Excretion into gastrointestinal tract of irinotecan lactone and carboxylate forms and their pharmacodynamics in rodents.

PURPOSE: To investigate the excretion of irinotecan hydrochloride (CPT-11) and its active metabolite, SN-38, into the gastrointestinal lumen via the biliary and/or intestinal membrane route after dosing with lactone and carboxylate forms of CPT-11, and to evaluate the toxic and antitumor effects of the two forms. METHODS: The excretions of CPT-11 and SN-38 were investigated by the in situ perfusion technique using rats. The incidence of delayed diarrhea was evaluated after i.v. dosing (60 mg/kg) with CPT-11 lactone and carboxylate forms for 4 days. Antitumor activity and changes in body weight were investigated in mice with Meth A tumors. RESULTS: The excretion of CPT-11 into bile was greater in dosing with CPT-11 carboxylate than that with its lactone form, whereas the exsorption across intestinal membrane was greater in dosing with CPT-11 lactone than that with its carboxylate form. Dosing with CPT-11 lactone dose-dependently inhibited the increase in tumor weights in Meth A tumor mice, whereas the dosing with its carboxylate form reduced the antitumor effect. CONCLUSIONS: The decreased antitumor effect caused by dosing with the CPT-11 carboxylate form could be due to less accumulation in the tissue including tumor cells resulting from the rapid elimination of the form in the body.

Cytogenetic identification of a hybrid owl monkey, Aotus nancymaae x Aotus lemurinus griseimembra.

A neonate male owl monkey (Aotus sp.) was identified cytogenetically as a hybrid after it failed to nurse and died. Phenotypically, the male parent possessed characteristics of the "gray-neck group," and G-banded karyotypes identified him as Aotus lemurinus griseimembra (2n = 53), heterozygous for the centric fusion of chromosomes 13 and 14. The female parent belonged to the "red-neck group" and was identified cytogenetically as Aotus nancymaae (2n = 54). The neonate hybrid had 2n = 54 chromosomes with 13 homologous pairs of autosomes, 26 nonhomologous autosomes, and XY sex chromosomes. Thirteen of the nonhomologous chromosomes represented the paternal complement, and 13 were from the maternal complement. Chromosomal rearrangements occurring between the karyotypes of A. l. griseimembra and A. nancymaae were believed to include two paracentric inversions, a reciprocal translocation, and two complex rearrangements involving pericentric inversion, telocentromeric fusion, and centromeric adjustment. Cytogenetic analyses are necessary to identify most Aotus taxa and thus should be utilized to pair chromosomally compatible animals and avoid interspecies hybridization.

Comparative cytogenetics of tapirs, genus tapirus (Perissodactyla, tapiridae).

Chromosomes of the four species of Tapirus were 2n = 52 in T. indicus, 2n = 76 in T. pinchaque, 2n = 80 in T. bairdii, and 2n = 80 in T. terrestris. The number of autosomal arms was 80-94. G-banded karyotypes indicated that a heterochromatic addition/deletion distinguished chromosomes 2 and 3 of T. bairdii and T. pinchaque, respectively. There were at least 13 conserved autosomes between the karyotypes of T. bairdii and T. terrestris, and at least 15 were conserved between T. bairdii and T. pinchaque. In G- and C-banded preparations, the X chromosomes of T. bairdii, T. indicus, and T. terrestris were identical, whereas the X chromosome of T. pinchaque differed from the X of the other species by a heterochromatic addition/deletion. The Y chromosome was a medium-sized to small acrocentric in T. bairdii, T. indicus, and T. pinchaque, but it was not positively identified in T. terrestris. There appeared to be fewer homologies between T. indicus and the three species occurring in Central and South America. Future cytogenetic studies of tapirs from the entire range of each of the four species might provide additional insight into their evolutionary biology and aid wildlife conservation efforts aimed at these threatened mammals.

Chromosomes of the antelope genus Kobus (Artiodactyla, Bovidae): karyotypic divergence by centric fusion rearrangements.

G- and C-banded karyotypes of four species of the genus Kobus were compared using the standard karyotype of Bos taurus. Chromosomal complements were 2n = 50-54 in K. ellipsiprymnus, 2n = 50 in K. kob, 2n = 48 in K. leche, and 2n = 52 in K. megaceros. The number of autosomal arms in all karyotypes was 58. Fifteen autosomal pairs were conserved among these four species, including the 1;19 and 2;25 centric fusions, and autosomal differences involved eight centric fusion rearrangements. Five centric fusions were each unique to a particular taxon: 3;10 (K. leche), 3;11 and 6;29 (K. kob), and 5;17 and 7;11 (K. ellipsiprymnus). The 4;7 fusion occurred in K. leche and K. megaceros, whereas the 5;13 fusion occurred in K. kob and K. leche; the 6;18 fusion was found in three species but was absent in K. kob. Differences between the X chromosomes of the four Kobus species were attributed to heterochromatic additions or deletions, and Y-chromosome differences may have been the result of pericentric inversion. G-banded karyotypes of putative K. l. leche and K. l. kafuensis appeared identical, as did C-banded karyotypes of the two subspecies. Karyotypes of K. e. ellipsiprymnus and K. e. defassa differed as a result of the 6;18 centric fusion, which was polymorphic in K. e. defassa, and the 7;11 centric fusion, which was polymorphic in K. e. ellipsiprymnus but absent in K. e. defassa. Several centric fusions were related by monobrachial chain-IV complexes; however, records of hybridization indicate that reproductive isolation between at least certain species of Kobus is incomplete. Karyotypic differences between K. ellipsiprymnus (including K. e. ellipsiprymnus and K. e. defassa), K. kob, K. leche, and K. megaceros support the validity of these taxa, as well as the need to manage them as separate populations.

A molecular cytogenetic analysis of the tribe Bovini (Artiodactyla: Bovidae: Bovinae) with an emphasis on sex chromosome morphology and NOR distribution.

Q-band comparisons were made among representative species of the four genera of the tribe Bovini (Bos, Bison, Bubalus, Syncerus) as well as to selected outgroup taxa representing the remaining two tribes of the subfamily Bovinae (nilgai, Boselaphini; eland, Tragelphini), the Bovidae subfamily Caprinae (domestic sheep) and the family Cervidae (sika deer and white-tailed deer). Extensive autosomal arm homologies were noted, but relatively few derivative character states were shared. Focus was then made on variation of the sex chromosomes and the chromosomal distribution of nucleolar organizer regions (NORs). Bovine BAC clones were used in molecular cytogenetic analyses to decipher rearrangements of the sex chromosomes, and a pocket gopher 28s ribosomal probe was used to map the chromosomal locations of nucleolar organizing regions (NORs). Some of the more noteworthy conclusions drawn from the comparative analysis were that: 1. The Bovidae ancestral X chromosome was probably acrocentric and similar to acrocentric X chromosomes of the Bovinae; 2. The domestic sheep acrocentric X is probably a derivative character state that unites non-Bovinae subfamilies; 3. Bos and Bison are united within the tribe Bovini by the presence of shared derivative submetacentric X chromosomes; 4. Sika and white-tailed deer X chromosomes differ by inversion from X chromosomes of the Bovinae; 5. The Bovini ancestral Y chromosome was probably a small acrocentric; 6. Bos taurus, B. gaurus and B. banteng share derivative metacentric Y chromosomes; 7. Syncerus and Bubalus are united by the acquisition of X-specific repetitive DNA sequence on their Y chromosomes; 8. Bovinae and Cervidae X chromosome centromere position varies without concomitant change in locus order. Preliminary data indicate that a knowledge of the chromosomal distribution of NORs among the Bovidae will prove to be phylogenetically informative.

Centric fusion differences among Oryx dammah, O. gazella, and O. leucoryx (Artiodactyla, Bovidae)

G- and C-banded karyotypes of the genus Oryx were compared using the standard karyotype of Bos taurus. Chromosomal complements were 2n = 56 in O. gazella gazella, 2n = 58 in O. g. beisa and O. g. callotis, 2n = 56-58 in O. dammah, and 2n = 57-58 in O. leucoryx. The number of autosomal arms in all karyotypes was 58. Nearly all variation in diploid number was the result of three independent centric fusions, but one 2n = 57 specimen of O. g. gazella deviated from the normal complement of 2n = 56 due to XXY aneuploidy. A 2;17 centric fusion was fixed in O. g. gazella, whereas O. g. beisa and O. g. callotis lacked this fusion and had indistinguishable karyotypes. Oryx dammah was polymorphic for a 2;15 centric fusion, and O. leucoryx was polymorphic for an 18;19 centric fusion. The five Oryx taxa shared a fixed 1;25 centric fusion; the small acrocentric element involved in the 1;25 fusion was identified by fluorescence in situ hybridization using a cosmid specific to Bos chromosome 25. The X and Y chromosomes were also conserved among the five taxa. Oryx g. gazella differed from the other Oryx species because of the fixed 2;17 centric fusion. This difference reflects an apparently longer period of geographic isolation between O. g. gazella and other populations of Oryx, and it is consistent with the classification of O. gazella and O. beisa as distinct species (see Kingdon, 1997). The lack of monobrachial relationships among the Oryx taxa indicates that sterility barriers between species have not developed. Viability of hybrid offspring constitutes a threat to captive breeding programs designed for endangered species conservation; in the case of Oryx, the 2;15, 2;17, and 18;19 metacentrics could serve as marker chromosomes for assessing hybridization between certain Oryx taxa.

Complex, compound inversion/translocation polymorphism in an ape: presumptive intermediate stage in the karyotypic evolution of the agile gibbon Hylobates agilis.

Karyotypic variation in five gibbon species of the subgenus Hylobates (2n = 44) was assessed in 63 animals, 23 of them wild born. Acquisition of key specimens of Hylobates agilis (agile gibbon), whose karyotype had been problematic due to unresolved structural polymorphisms, led to disclosure of a compound inversion/translocation polymorphism. A polymorphic region of chromosome 8 harboring two pericentric inversions, one nested within the other, was in turn bissected by one breakpoint of a reciprocal translocation. In double-inversion + translocation heterozygotes, the theoretical meiotic pairing configuration is a double inversion loop, with four arms of a translocation quadrivalent radiating from the loop. Electron-microscopic analysis of synaptonemal complex configurations consistently revealed translocation quadrivalents but no inversion loops. Rather, nonhomologous pairing was evident in the inverted region, a condition that should preclude crossing over and the subsequent production of duplication-deficiency gametes. This is corroborated by the existence of normal offspring of compound heterozygotes, indicating that fertility may not be reduced despite the topological complexity of this polymorphic system. The distribution of inversion and translocation morphs in these taxa suggests application of cytogenetics in identifying gibbon specimens and avoiding undesirable hybridization in captive breeding efforts.

Chromosomal rearrangements in a Somali wild ass pedigree, Equus africanus somaliensis (Perissodactyla, Equidae).

Chromosome analyses were conducted on 15 animals in a pedigree of Somali wild ass, Equus africanus somaliensis. G- and C-banded karyotypes are presented for the first time on this endangered species. The diploid number ranged from 62 to 64. Numerical chromosomal variation was the result of a centric fission which was accompanied by a heterochromatic deletion. The fission polymorphism involved acrocentric elements 19 and 21 as determined by G-banding. These autosomes are homologous to those involved in centric fission/fusion polymorphisms in other equids: E. asinus (domestic donkey), E. hemionus (onager), E. kiang (kiang), and E. burchelli (common zebra). Banding analyses also revealed a paracentric inversion polymorphism in submetacentric chromosome pair 2 of E. a. somaliensis. Both the centric fission and paracentric inversion polymorphisms involved heterochromatic regions. One individual was found to be heterozygous for two de novo chromosomal rearrangements: a centric fission (involving acrocentric elements 19 and 21) and a heterochromatic deletion of chromosome 2.

Centric fusion polymorphisms in waterbuck (Kobus ellipsiprymnus).

Twenty-six captive individuals of the ellipsiprymnus subspecies group of Kobus ellipsiprymnus were found to have chromosomal complements of 2n = 50-52 (FN = 61-62), and 26 of the defassa subspecies group, including three specimens from Lake Nakuru National Park, Kenya, had complements of 2n = 53-54 (FN = 62). G-banded karyotypes that were numbered according to the standard karyotype of Bos taurus revealed that variation in diploid number was the result of polymorphism for two independent centric (Robertsonian) fusions. The ellipsiprymnus group was polymorphic for a 7;11 centric fusion. Both elements of chromosome pairs 7 and 11 were fused in fusion homozygotes (2n = 50); in fusion heterozygotes (2n = 51), only one element of each pair was fused. The 7;11 fusion was lacking in specimens with 2n = 52. The defassa group was polymorphic for a 6;18 centric fusion; individuals were either heterozygous for the fusion (2n = 53) or lacking it (2n = 54). There were no defassa group individuals that were homozygous for the 6;18 fusion (2n = 52), but this may be a sampling artifact. The 6;18 fusion was fixed in the ellipsiprymnus group, whereas the 7;11 fusion was absent in the defassa group. In G- and C-banded karyotypes, all autosomal arms and the X chromosomes of the two subspecies groups appeared to be completely homologous. However, the Y chromosome was acrocentric in the ellipsiprymnus group and submetacentric in the defassa group, possibly the result of a pericentric inversion. Fixed chromosomal differences between the two subspecies groups reflect a period of supposed geographic isolation during which time they diverged genetically and phenotypically, and the centric fusion polymorphisms raise the possibility of reduced fertility in hybrids. These data, in conjunction with phenotypic and mitochondrial DNA data, suggest to us that populations of the ellipsiprymnus and defassa groups should be managed separately.

A karyotypic analysis of nilgai, Boselaphus tragocamelus (Artiodactyla: Bovidae).

A combination of chromosomal banding and fluorescence in situ hybridization (FISH) was used to characterize the karyotype of Boselaphus tragocamelus (nilgai) relative to the domestic cattle standard karyotype. G-, Q- and C-band karyotypes of nilgai are presented, and the chromosomal complement of nilgai is determined to be 2n=46 (female FN=60, male FN=59; NAA=56), consistent with previous reports for the species. Comparisons with cattle identified extensive monobrachial homologies with some noteworthy exceptions. Chromosome 25 is centrically fused to 24, and chromosome 16 is acrocentric. Both appear to have additional pericentromeric material not seen in the equivalent cattle acrocentrics. This pericentromeric chromatin may be the result of de novo additions or translocation of pericentromeric material from chromosome 6, which is shown to be centrically fused to 13 but is only about two-thirds the length of cattle 6. Comparisons with cattle demonstrated that nilgai chromosome 17 has undergone a paracentric inversion and that chromosome 20 has two blocks of interstitial constitutive heterochromatin. The identities of both chromosomes were confirmed by chromosomal FISH. Furthermore, chromosomal banding and FISH were used to determine that autosome 14 has been fused to the ancestral X and Y of nilgai to form compound neo-X and -Y chromosomes. Additional FISH analyses were conducted to confirm other proposed chromosome homologies and to identify nucleolar organizing regions within the nilgai complement.