Native North American Populations Research Articles

Evolution of "invasion syndrome" in invasive goldenrod is not constrained by genetic trade-offs.

A suite of plant traits is thought to make weed populations highly invasive, including vigorous growth and reproduction, superior competitive ability, and high dispersal ability. Using a breeding design and a common garden experiment, we tested whether such an "invasion syndrome" has evolved in an invasive range of Solidago altissima, and whether the evolution is likely to be genetically constrained. We found an overall shift in invasive phenotypes between native North American and invasive Japanese populations. The invasive populations were taller and produced more leaves, suggesting a superior ability to exploit limited resources. The populations also produced more allelopathic compounds that can suppress competitor growth. Finally, invasive populations produced more seeds, which are smaller and are released from a greater height, indicating a potential for superior dispersal ability than the native populations. Quantitative genetics analyses found a large amount of additive genetic variation in most focal traits across native and invasive populations, with no systematic differences in its magnitude between the ranges. Genetic covariances among three traits representing invasion strategies (leaf mass, polyacetylene concentration and seed size) were small. The R metric, which measures the effect of genetic covariances on the rate of adaptation, indicated that the covariance neither constrains nor accelerates concerted evolution of these traits. The results suggest that the invasion syndrome in S. altissima has evolved in the novel range due to ample additive genetic variation, and relatively free from genetic trade-offs.

Botany is the root and the future of invasion biology.

Botany is the root and the future of invasion biology.

Genetic diversity in introduced Douglas-fir and its natural regeneration in Central Europe

AbstractSince its first introduction in the 19th century, Douglas-fir has become the economically most important non-native forest tree species in Central European countries. Many of these planted forests are important seed sources and/or exhibit natural regeneration. Thus, it is important to assess (1) the genetic diversity of the mature stands and (2) if the genetic diversity can be passed on to the next generations. In order to address these issues, we genotyped mature Douglas-fir individuals and natural regeneration from >100 native and non-native populations using nuclear microsatellite markers. We compared the genetic diversity of native North American populations with mature Douglas-fir populations in Central Europe. The results show that genetic diversity did not differ significantly between European populations and the assigned native origin. Using a subset of 36 sites from Central Europe, we detected a significant reduction in the genetic diversity of adult versus naturally regenerated juvenile trees, indicating a bottleneck effect in the next generation of European Douglas-fir stands. The main reason may be that the mature European Douglas-fir stands are highly fragmented and thus the stand size is not adequate for transmitting the genetic diversity to the next generation. This should be taken into account for the commercial harvesting of seed stands. Seed orchards may offer a potential alternative in providing high quality and genetically diverse reproductive material.

Context‐dependent induction of allelopathy in plants under competition

Some plants use allelopathy to compete against neighbouring plants, and the ability to induce allelopathic compound production in response to competition is hypothesized to be adaptive, as plants can save costs of metabolite production in the absence of competitors. However, whether plants induce allelopathy has rarely been explored so far.We studied the inducibility of polyacetylenes – putative allelopathic compounds in Solidago altissima – in response to competition. Polyacetylenes were found in natural soil surrounding S. altissima patches within the range of concentration known to inhibit competitor growth. Individual S. altissima plants with higher polyacetylene concentration in roots suppressed the growth of the competitor plants more, suggesting that root polyacetylene levels proximate plants’ allelopathic capacity. Competition induced polyacetylenes in a context‐dependent manner: Whereas introduced Japanese and Australian populations of S. altissima had higher constitutive concentration of polyacetylenes than the native North American populations, inducibility was observed only in Australian plants, where the population is still at an early stage of invasion. Also, induction became more prominent under nutrient depletion, where enhanced allelopathy may be particularly beneficial for suppressing a competitor's exploitative capacity. Finally, we found weak evidence for a tradeoff between constitutive and induced polyacetylenes.The observed patterns suggest that allelopathic plants could respond to competition by inducing allelochemical production, but the benefit of such plasticity may vary across time and space. Shifts in competitor communities in introduced range over time may shape plant's plastic responses to competition, while variation in resource availability may alter competitive environment to influence the degree to which plants induce allelopathy.

Patterns of genetic variation reflect multiple introductions and pre-admixture sources of common ragweed (Ambrosia artemisiifolia) in China

Ambrosia artemisiifolia is native to North America but has become a worldwide invasive weed. It was introduced to China more than 80 years ago and has spread into 20 provinces since then. To assess the population structure of A. artemisiifolia in China and whether this invasion involved a single event or multiple events, we investigated patterns of genetic variation for three chloroplast DNA intergenic spacer regions, a nrITS region and five microsatellite loci. Our dataset consists of 370 individuals from 19 sites throughout China. We compared their cpDNA-haplotypes to those published for native North American populations. The distribution of cpDNA-haplotypes indicates that A. artemisiifolia was introduced to China multiple times from different source regions. The numbers of alleles in Chinese populations were not significantly lower than in native populations. Both nrITS-haplotypes and microsatellite alleles showed that there was no evidence for a genetic bottleneck. Four populations were genetically well separated from the other 15 populations. However, the absence of isolation by distance, and the low levels of genetic differentiation and gene flow among the other 15 population suggest that most populations in China come from pre-admixed populations. To find the exact source regions of the Chinese populations, more samples from the native region and other invaded regions will be necessary. Nevertheless, our study provides important insights into the genetic background of A. artemisiifolia invasion in China.

Genetic diversity and differentiation of introduced red oak (Quercus rubra) in Germany in comparison with reference native North American populations

Northern red oak (Quercus rubra) was introduced to Europe in the late seventeenth century and has since become the most important deciduous non-native tree species in Germany. Despite its importance, little is known about the origin and patterns of genetic variation in German red oak stands. To be able to make recommendations regarding the adaptive potential of red oak stands, which might be related to their origin and the selection of provenances, with respect to climate change, a better understanding of the genetic diversity and structure of German red oak stands is needed. Individuals from 62 populations in Germany and North America were genotyped at five chloroplast microsatellite loci to characterize chloroplast haplotype diversity and geographic structure. Compared to reference populations from the natural distribution range, German red oak stands demonstrated a relatively low genetic differentiation among populations and represented only a fraction of the haplotype diversity found in North America. For several stands located in southern Germany, considerably higher haplotype diversity than in other German stands was found. While most German stands showed signatures of founder effects, the diversity of stands in southern Germany might have been increased due to admixture and multiple introductions of different North American provenances. Overall, we conclude that German stands originated from a limited geographic area, possibly located in the northern part of the native distribution range.

A common environment experiment reveals plastic and genetic contributions to the fast life‐history strategy of an invasive fish

AbstractInvasive species often exhibit a suite of life‐history traits that promote rapid population growth, including early age and small size at maturation, and high reproductive investment. The common expression of these “fast” life‐history traits in invasive populations could be the result of plastic and/or genetic responses to the non‐native environment, or in response to the process of range expansion. To determine the relative importance of plastic and genetic contributions to the expression of life‐history traits, we reared two native Canadian and two invasive Spanish populations of Pumpkinseed sunfish (Lepomis gibbosus) in a common environment in central Ontario, Canada. In the wild, European Pumpkinseed tend to exhibit faster juvenile growth rates, younger age and smaller size at maturity, and higher reproductive investment than native North American populations. When reared in a common environment, both native and invasive populations exhibited similar juvenile growth rates, and similar age and size at maturity, suggesting that the differences seen among wild populations are a plastic response to the warmer non‐native environment. However, reproductive investment was consistently higher in the Spanish populations regardless of rearing environment, suggesting a genetic difference in reproductive investment between native and invasive populations. Selection for greater reproductive investment in non‐native Pumpkinseed may have contributed to their widespread success in Europe.

Influence of invasion history on rapid morphological divergence across island populations of an exotic bird.

There is increasing evidence that exotic populations may rapidly differentiate from those in their native range and that differences also arise among populations within the exotic range. Using morphological and DNA‐based analyses, we document the extent of trait divergence among native North American and exotic Hawaiian populations of northern cardinal (Cardinalis cardinalis). Furthermore, using a combination of historical records and DNA‐based analyses, we evaluate the role of founder effects in producing observed trait differences. We measured and compared key morphological traits across northern cardinal populations in the native and exotic ranges to assess whether trait divergence across the Hawaiian Islands, where this species was introduced between 1929 and 1931, reflected observed variation across native phylogeographic clades in its native North America. We used and added to prior phylogenetic analyses based on a mitochondrial locus to identify the most likely native source clade(s) for the Hawaiian cardinal populations. We then used Approximate Bayesian Computation (ABC) to evaluate the role of founder effects in producing the observed differences in body size and bill morphology across native and exotic populations. We found cardinal populations on the Hawaiian Islands had morphological traits that diverged substantially across islands and overlapped the trait space of all measured native North American clades. The phylogeographic analysis identified the eastern North American clade (C. cardinalis cardinalis) as the most likely and sole native source for all the Hawaiian cardinal populations. The ABC analyses supported written accounts of the cardinal's introduction that indicate the original 300 cardinals shipped to Hawaii were simultaneously and evenly released across Hawaii, Kauai, and Oahu. Populations on each island likely experienced bottlenecks followed by expansion, with cardinals from the island of Hawaii eventually colonizing Maui unaided. Overall, our results suggest that founder effects had limited impact on morphological trait divergence of exotic cardinal populations in the Hawaiian archipelago, which instead reflect postintroduction events.

Isolated populations of Eurytemora americana Williams (Crustacea, Copepoda) in the White Sea rock pools—postglacial relicts or anthropogenic invasions?

Molecular-genetic study of isolated populations of Eurytemora in the White Sea showed its undoubted proximity to the native North American populations of Eurytemora americana, Williams 1906. Genetic tests show the recent colonization of the White Sea rock pools by initially small amount of individuals of E. americana. Existing published data on the presence of isolated Atlantic and Pacific species in the Arctic region are mostly explained by historical reasons: preservation of relict populations formerly inhabited the Holarctic and separated by processes of formation and melting of glaciers. Finding of E. americana in the White Sea allowed us to suppose a possible reason of formation of these isolates: transportation with ship ballast waters during the period of intensive cargo in the Second World War along the edge of the Arctic seas.

The role of environmental factors and genetic diversity on colonization success of a non-native fish, Lepomis gibbosus from western part of Turkey

Non-native freshwater fishes may become one of the dominant species in still and slow running waters and may impact whole ecosystem. One such species is the pumpkinseed Lepomis gibbosus, which has become widely distributed throughout Western Europe occurring primarily lakes and reservoirs. Although the species is known to have existed at least for four decades and expanded its distribution range in Turkish waters widely, virtually there is no study on the factors behind the species' establishment success. We hypothesized high environmental tolerance as well as high population structuring result in more successful establishment of pumpkinseed in new environments. We sampled three reservoirs, one canal and one stream located longitudinally along western part of Turkey to test this hypothesis. Optima (uk) and tolerance (tk) levels of the species were calculated with weighted averaging values for several measured environmental factors. Invasiveness of pumpkinseed populations was determined with the model proposed earlier examining the relationship between juvenile growth and age at maturity. The phylogenetic network of mitochondrial haplotypes was constructed for Turkish populations and other populations of pumpkinseed from native (North America) and introduced range (Europe). Potential invasiveness of pumpkinseed populations in Turkey was categorized as invasive in one location (Sarıçay stream) and transitional in other locations. Environmental tolerance values were higher in southern populations. Likewise phylogenetic network showed that the Turkish populations have unique haplotypes, which were originated from native North American populations suggesting a high differentiation in haplotypes. Results clearly demonstrate the effect of environmental factors on genetic structure and the establishment success of pumpkinseed.

Reciprocal interactions between native and introduced populations of common milkweed, Asclepias syriaca, and the specialist aphid, Aphis nerii

Following its introduction into Europe (EU), the common milkweed (Asclepias syriaca) has been free of most specialist herbivores that are present in its native North American (NA) range, except for the oleander aphid Aphis nerii. We compared EU and NA populations of A. nerii on EU and NA milkweed populations to test the hypothesis that plant–insect interactions differ on the two continents. First, we tested if herbivore performance is higher on EU plants than on NA plants, because the former have escaped most of their herbivores and have perhaps been selected for lower defence levels following introduction. Second, we compared two A. nerii lines (one from each continent) to test whether genotypic differences in the herbivore may influence species interactions in plant–herbivore communities in the context of species introductions. The NA population of A. nerii developed faster, had higher fecundity and attained higher population growth rates than the EU population. There was no overall significant continental difference in aphid resistance between the plants. However, milkweed plants from EU supported higher population growth rates and faster development of the NA line of A. nerii than plants from NA. In contrast, EU aphids showed similar (low) performance across plant populations from both continents. In a second experiment, we examined how chewing herbivores indirectly mediate interactions between milkweeds and aphids, and induced A. syriaca plants from each continent by monarch caterpillars (Danaus plexippus) to compare the resulting changes in plant quality on EU aphid performance. As specialist chewing herbivores of A. syriaca are only present in NA, we expected that plants from the two continents may affect aphid growth in different ways when they are challenged by a specialist chewing herbivore. Caterpillar induction decreased aphid developmental times on NA plants, but not on EU plants, whereas fecundity and population growth rates were unaffected by induction on both plant populations. The results show that genetic variation in the plants as well as in the herbivores can determine the outcome of plant–herbivore interactions.

Morphological change and phenotypic plasticity in native and non-native pumpkinseed sunfish in response to competition.

Non-indigenous species are oftentimes exposed to ecosystems with unfamiliar species, and organisms that exhibit a high degree of phenotypic plasticity may be better able to contend with the novel competitors that they may encounter during range expansion. In this study, differences in morphological plasticity were investigated using young-of-year pumpkinseed sunfish (Lepomis gibbosus) from native North American and non-native European populations. Two Canadian populations, isolated from bluegill sunfish (L. macrochirus) since the last glaciation, and two Spanish populations, isolated from bluegill since their introduction in Europe, were reared in a common environment using artificial enclosures. Fish were subjected to allopatric (without bluegill) or sympatric (with bluegill) conditions, and differences in plasticity were tested through a MANOVA of discriminant function scores. All pumpkinseed populations exhibited dietary shifts towards more benthivorous prey when held with bluegill. Differences between North American and European populations were observed in body dimensions, gill raker length and pelvic fin position. Sympatric treatments induced an increase in body width and a decrease in caudal peduncle length in native fish; non-native fish exhibited longer caudal peduncle lengths when held in sympatry with bluegill. Overall, phenotypic plasticity influenced morphological divergence less than genetic factors, regardless of population. Contrary to predictions, pumpkinseeds from Europe exhibited lower levels of phenotypic plasticity than Canadian populations, suggesting that European pumpkinseeds are more canalized than their North American counterparts.

Clinicopathologic Features of Adult T-cell Leukemias/Lymphomas at a North American Tertiary Care Medical Center

Human T-cell lymphotropic virus type 1 is associated with adult T-cell leukemia/lymphoma (ATLL). Published series of ATLLs seen at a United States medical institution are rare. We present the features of 4 ATLLs diagnosed at our North American tertiary care medical center from 1990 to 2012. Despite the absence of a history of origin from an endemic region, all our ATLLs demonstrated evidence of human T-cell lymphotropic virus type 1 infection. Central nervous system (CNS) involvement by ATLL was uncommon in our series, and represented only 1.6% (1/64) of all CNS B-cell or T-cell lymphomas diagnosed over a 20+ year period at our institution. Review of the medical literature reveals that the majority of CNS-involved ATLLs present with the lymphoma or acute subtype, and complete remission is difficult to achieve in these cases. CNS involvement frequently occurs with a systemic disease, which carries an aggressive clinical course with poor prognosis. In addition, CNS involvement by ATLL can be the initial presentation or seen with relapsed disease, can be the only site or be associated with other tissue sites of involvement, and may manifest with variable clinical signs/symptoms. Our retrospective study reveals that ATLLs are rare mature T-cell lymphomas in a native North American population, but the clinical and histopathologic features of ATLLs from this nonendemic region are similar to those seen from other endemic regions. Early recognition of these rare ATLLs involving uncommon sites, such as the CNS, will help optimize treatment for these infrequent mature T-cell lymphomas.

The History of Reed Canarygrass in North America: Persistence of Natives among Invading Eurasian Populations

ABSTRACTReed canarygrass (Phalaris arundinaceaL.) is a major invader in North American wetlands that has also been planted as a forage crop for nearly a century. While the species is native to both North America and Eurasia, the geographic origin of invaders has remained unknown due to the previous inability to distinguish North American and Eurasian populations. The goals of this study were to (i) identify extant native North American populations if they exist, (ii) determine the geographic origin of present‐day populations in North America, and (iii) compare the diversity among Eurasian and North American populations to determine whether there is evidence of a genetic bottleneck in North America. A total of 987 individuals from 373 accessions were investigated using 15 microsatellite markers. A subset of 37 individuals was sequenced at four chloroplast regions. The population structure of the species was inferred using Bayesian clustering approaches and a median joining tree. Native North American populations of reed canarygrass are still present in North America. However, only 3 of the 235 North American accessions sampled were of North American origin. All Eurasian subpopulations were present in North America and invading populations in North America were as diverse as populations in the Eurasian range. The Eurasian populations are far more common in North American wetlands today and suggest introduced Eurasian populations are more invasive than their North American counterparts although the mechanisms for the differences in invasibility remain a mystery.

European Invasion of North American Pinus strobus at Large and Fine Scales: High Genetic Diversity and Fine-Scale Genetic Clustering over Time in the Adventive Range

BackgroundNorth American Pinus strobus is a highly invasive tree species in Central Europe. Using ten polymorphic microsatellite loci we compared various aspects of the large-scale genetic diversity of individuals from 30 sites in the native distribution range with those from 30 sites in the European adventive distribution range. To investigate the ascertained pattern of genetic diversity of this intercontinental comparison further, we surveyed fine-scale genetic diversity patterns and changes over time within four highly invasive populations in the adventive range.ResultsOur data show that at the large scale the genetic diversity found within the relatively small adventive range in Central Europe, surprisingly, equals the diversity found within the sampled area in the native range, which is about thirty times larger. Bayesian assignment grouped individuals into two genetic clusters separating North American native populations from the European, non-native populations, without any strong genetic structure shown over either range. In the case of the fine scale, our comparison of genetic diversity parameters among the localities and age classes yielded no evidence of genetic diversity increase over time. We found that SGS differed across age classes within the populations under study. Old trees in general completely lacked any SGS, which increased over time and reached its maximum in the sapling stage.ConclusionsBased on (1) the absence of difference in genetic diversity between the native and adventive ranges, together with the lack of structure in the native range, and (2) the lack of any evidence of any temporal increase in genetic diversity at four highly invasive populations in the adventive range, we conclude that population amalgamation probably first happened in the native range, prior to introduction. In such case, there would have been no need for multiple introductions from previously isolated populations, but only several introductions from genetically diverse populations.

The Value of Studying Clinical and Serologic Phenotypes in North American Native Populations with Autoimmune Disease

In the last 10–15 years there has been increasing awareness of a high rheumatic disease burden in North American Natives (NAN), resulting in a growing number of studies describing clinical and serological phenotypes in these populations. But what is the value of these largely descriptive studies? In this issue of The Journal , Bacher and colleagues add to the body of literature on rheumatic disease in NAN populations, describing the manifestations and symptoms of scleroderma (systemic sclerosis; SSc) in a group of 71 Native Canadians1. This is the largest NAN cohort ever described with this relatively rare disease, and the authors suggest possible differences in the phenotype of SSc compared to white populations. Variability in the phenotypic expression of many autoimmune diseases between different ethnicities has long been recognized; in NAN populations autoimmune disease is generally recognized to be severe2,3,4. At the very least, descriptions of disparate burdens of disease can help guide public health policy, and direct increased health resources to affected ethnic groups. Ethnicity, however, is a complex concept, which includes racial designations or genotypic groupings, but transcends them, representing instead the aggregate of cultural practices, lifestyle patterns, social influences, religious pursuits, and racial characteristics that shape the distinctive identity of a community5. Autoimmune diseases, such as SSc and systemic lupus erythematosus (SLE), are known to arise from a complex interaction between genetic, environmental, socioeconomic, cultural, and … Address correspondence to Dr. Peschken; E-mail: cpeschken{at}exchange.hsc.mb.ca

Systemic Sclerosis in Canada’s North American Native Population: Assessment of Clinical and Serological Manifestations

Certain North American Native (NAN) populations are known to have higher rates of systemic sclerosis (SSc) compared to non-NAN; however, little is known of the specific disease characteristics in this population in Canada. This study compares the clinical and serological manifestations of SSc in NAN and white patients. This cross-sectional, multicenter study included subjects enrolled in the Canadian Scleroderma Research Group registry between September 2004 and June 2012. Subjects were evaluated with complete medical histories, physical examinations, and self-questionnaires. Ethnicity was defined by self-report. Disease characteristics were compared between NAN and white patients and multivariate analyses were performed to determine the independent association between ethnicity and various clinical manifestations. Of 1278 patients, 1038 (81%) were white, 71 (6%) were NAN, and 169 (13%) were classified as non-white/non-NAN. There were important differences between NAN and white subjects with SSc. In multivariate analysis adjusting for socioeconomic differences and smoking status, NAN ethnicity was an independent risk factor for the severity of Raynaud phenomenon and more gastrointestinal symptoms, and was associated with a nonsignificant increase in the presence of digital ulcers. NAN patients with SSc have a distinct clinical phenotype. Our study provides a strong rationale to pursue further research into genetic and environmental determinants of SSc.

Occlusal variation in the Zuni: a pre-contact North American population

Abstract Background: Reports of occlusal variation in ancient populations consistently show a low prevalence of malocclusion coupled with heavy attritive wear. Materials and methods: The dentitions of 28 individual remains from a pre-contact native North American population were examined and the extent and nature of occlusal variation recorded. Results: The prevalence of malocclusion was low (mean IOTN = 2.14) and where crowding existed, was limited to intra-arch variability as opposed to inter-arch discrepancies. Increased overbites, overjets and other classical features of Class II malocclusions were almost entirely absent. These findings suggest that the ideal mutually protected Class I occlusion occurs only as a transient juvenile arrangement in nature, where, due to rapid attrition following establishment of the occlusion, there is an increasing tendency towards mild Class III, edge-to-edge incisor and buccal segment relationships. Conclusion: Whilst of limited therapeutic benefit, an understanding of the anthropology of malocclusion provides an insightful perspective, and suggests that function may be of more importance than heredity in its aetiology. It is suggested that future occlusal studies in ancestral populations would benefit from the use of a standard methodology.

American Indian Sainthood and the Catholic Church

Recent decades have witnessed Indian participation diminish within Christian churches. This trend underscores the importance of Kateri Tekakwitha's canonization on October 21, 2012. It was a long overdue acknowledgement of the sanctity that is just as evident within Native North American populations as it is among others. With Indian America now having a woman saint, a male counterpart ought to be close behind her. Nicholas Black Elk is the pre-eminent candidate for this honor. Probably better known than Kateri, Black Elk's popular "life story" was published in 1932, and reprinted many times in at least eleven languages. It was not known until the end of the twentieth century, however, that most of the man's life had gone unreported. Unknown to readers was that Black Elk had been a devout Catholic catechist for close to fifty years. It was this role for which his people held him in high esteem (400 of them baptized because of his missionary labors). In the event Black Elk is canonized, the church can be indicted for doing too little too late. Had it acted more quickly, the church might have provided balance to an Indian cultural resurgence that included heavy doses of anti-Christian rhetoric that encouraged many to forsake affiliation with faith communities.

Pregnancy and the Risk of Rheumatoid Arthritis in a Highly Predisposed North American Native Population

To examine reproductive history and rheumatoid arthritis (RA) risk in a highly predisposed population of North American Natives (NAN) with unique fertility characteristics. The effect of pregnancy on the risk of RA was examined by comparing women enrolled in 2 studies: a study of RA in NAN patients and their unaffected relatives; and NAN patients with RA and unrelated healthy NAN controls enrolled in a study of autoimmunity. All participants completed questionnaires detailing their reproductive history. Patients with RA (n = 168) and controls (n = 400) were similar overall in age, education, shared epitope frequency, number of pregnancies, age at first pregnancy, smoking, and breastfeeding history. In multivariate analysis, for women who had ≥ 6 births the OR for developing RA was 0.43 (95% CI 0.21-0.87) compared with women who had 1-2 births (p = 0.046); for women who gave birth for the first time after age 20 the OR for developing RA was 0.33 (95% CI 0.16-0.66) compared with women whose first birth occurred at age ≤ 17 (p = 0.001). The highest risk of developing RA was in the first postpartum year (OR 3.8; 95% CI 1.45-9.93) compared with subsequent years (p = 0.004). In this unique population, greater parity significantly reduced the odds of RA; an early age at first birth increased the odds, and the postpartum period was confirmed as high risk for RA onset. The protective effect of repeated exposure to the ameliorating hormonal and immunological changes of pregnancy may counterbalance the effect of early exposure to the postpartum reversal of these changes.