Flowering plant reproduction.

Flowering plants, also known as angiosperms, emerged approximately 150 to 200 million years ago. Since then, they have undergone rapid and extensive expansion, now encompassing around 90% of all land plant species. The remarkable diversification of this group has been a subject of in-depth investigations, and several evolutionary innovations have been proposed to account for their success. In this primer, we will specifically focus on one such innovation: the advent of seeds containing endosperm.

Helitrons: genomic parasites that generate developmental novelties.

Helitrons, classified as DNA transposons, employ rolling-circle intermediates for transposition. Distinguishing themselves from other DNA transposons, they leave the original template element unaltered during transposition, which has led to their characterization as 'peel-and-paste elements'. Helitrons possess the ability to capture and mobilize host genome fragments, with enormous consequences for host genomes. This review discusses the current understanding of Helitrons, exploring their origins, transposition mechanism, and the extensive repercussions of their activity on genome structure and function. We also explore the evolutionary conflicts stemming from Helitron-transposed gene fragments and elucidate their domestication for regulating responses to environmental challenges. Looking ahead, further research in this evolving field promises to bring interesting discoveries on the role of Helitrons in shaping genomic landscapes.

Molecular basis and evolutionary drivers of endosperm-based hybridization barriers.

The endosperm, a transient seed tissue, plays a pivotal role in supporting embryo growth and germination. This unique feature sets flowering plants apart from gymnosperms, marking an evolutionary innovation in the world of seed-bearing plants. Nevertheless, the importance of the endosperm extends beyond its role in providing nutrients to the developing embryo by acting as a versatile protector, preventing hybridization events between distinct species and between individuals with different ploidy. This phenomenon centers on growth and differentiation of the endosperm and the speed at which both processes unfold. Emerging studies underscore the important role played by type I MADS-box transcription factors, including the paternally expressed gene PHERES1. These factors, along with downstream signaling pathways involving auxin and abscisic acid, are instrumental in regulating endosperm development and, consequently, the establishment of hybridization barriers. Moreover, mutations in various epigenetic regulators mitigate these barriers, unveiling a complex interplay of pathways involved in their formation. In this review, we discuss the molecular underpinnings of endosperm-based hybridization barriers and their evolutionary drivers.

[Diagnostic Imaging during pregnancy of the dog]. / Bildgebende Diagnostik im Rahmen der Trächtigkeit beim Hund.

Diagnostic imaging represents an important part during modern pregnancy management of the dog. Ultrasound has become the modality of choice for an early pregnancy diagnosis, enabling a verification as early as 17-20 days after LH surge. Furthermore an evaluation of fetal viability and development is possible as well as the depiction of malformations and an early assessment of fetal stress. Doppler imaging may also be of supporting value in this context . Using fetal maturity as well as measurements of fetal and extrafetal structures, a calculation of the gestational age and therefore the time of parturition is achievable. Radiographs of the caudal abdomen enable pregnancy diagnosis from the 43rd day after LH surge onward. In addition, an assessment of fetal size as well as litter size is possible. Both modalities allow for complementary statements concerning possible labor complications and possess supporting value concerning the decision of necessary obstetric intervention and further management in patients with dystocia.

Plant development: How to kill the endosperm.

Using molecular markers and genetic analysis of mutant phenotypes, a new study reveals that endosperm elimination in plant seeds is under control of the programmed cell death pathway.

Updated Phylogeny and Protein Structure Predictions Revise the Hypothesis on the Origin of MADS-box Transcription Factors in Land Plants.

MADS-box transcription factors (TFs), among the first TFs extensively studied, exhibit a wide distribution across eukaryotes and play diverse functional roles. Varying by domain architecture, MADS-box TFs in land plants are categorized into Type I (M-type) and Type II (MIKC-type). Type I and II genes have been considered orthologous to the SRF and MEF2 genes in animals, respectively, presumably originating from a duplication before the divergence of eukaryotes. Here, we exploited the increasing availability of eukaryotic MADS-box sequences and reassessed their evolution. While supporting the ancient duplication giving rise to SRF- and MEF2-types, we found that Type I and II genes originated from the MEF2-type genes through another duplication in the most recent common ancestor (MRCA) of land plants. Protein structures predicted by AlphaFold2 and OmegaFold support our phylogenetic analyses, with plant Type I and II TFs resembling the MEF2-type structure, rather than SRFs. We hypothesize that the ancestral SRF-type TFs were lost in the MRCA of Archaeplastida (the kingdom Plantae sensu lato). The retained MEF2-type TFs acquired a Keratin-like domain and became MIKC-type before the divergence of Streptophyta. Subsequently in the MRCA of land plants, M-type TFs evolved from a duplicated MIKC-type precursor through loss of the Keratin-like domain, leading to the Type I clade. Both Type I and II TFs expanded and functionally differentiated in concert with the increasing complexity of land plant body architecture. The recruitment of these originally stress-responsive TFs into developmental programs, including those underlying reproduction, may have facilitated the adaptation to the terrestrial environment.

Endosperm cellularization failure induces a dehydration-stress response leading to embryo arrest.

The endosperm is a nutritive tissue supporting embryo growth in flowering plants. Most commonly, the endosperm initially develops as a coenocyte (multinucleate cell) and then cellularizes. This process of cellularization is frequently disrupted in hybrid seeds generated by crosses between different flowering plant species or plants that differ in ploidy, resulting in embryo arrest and seed lethality. The reason for embryo arrest upon cellularization failure remains unclear. In this study, we show that triploid Arabidopsis thaliana embryos surrounded by uncellularized endosperm mount an osmotic stress response that is connected to increased levels of abscisic acid (ABA) and enhanced ABA responses. Impairing ABA biosynthesis and signaling aggravated triploid seed abortion, while increasing endogenous ABA levels as well as the exogenous application of ABA-induced endosperm cellularization and suppressed embryo growth arrest. Taking these results together, we propose that endosperm cellularization is required to establish dehydration tolerance in the developing embryo, ensuring its survival during seed maturation.

Meiocyte size is a determining factor for unreduced gamete formation in Arabidopsis thaliana.

Polyploidy, the presence of more than two sets of chromosomes within a cell, is a widespread phenomenon in plants. The main route to polyploidy is considered through the production of unreduced gametes that are formed as a consequence of meiotic defects. Nevertheless, for reasons poorly understood, the frequency of unreduced gamete formation differs substantially among different plant species. The previously identified meiotic mutant jason (jas) in Arabidopsis thaliana forms about 60% diploid (2n) pollen. JAS is required to maintain an organelle band as a physical barrier between the two meiotic spindles, preventing previously separated chromosome groups from uniting into a single cell. In this study, we characterized the jas suppressor mutant telamon (tel) that restored the production of haploid pollen in the jas background. The tel mutant did not restore the organelle band, but enlarged the size of male jas tel meiocytes, suggesting that enlarged meiocytes can bypass the requirement of the organelle band. Consistently, enlarged meiocytes generated by a tetraploid jas mutant formed reduced gametes. The results reveal that meiocyte size impacts chromosome segregation in meiosis II, suggesting an alternative way to maintain the ploidy stability in meiosis during evolution.

Genomic imprinting regulates establishment and release of seed dormancy.

Seed dormancy enables plant seeds to time germination until environmental conditions become favorable for seedling survival. This trait has high adaptive value and is of great agricultural relevance. The endosperm is a reproductive tissue formed after fertilization that in addition to support embryo growth has major roles in establishing seed dormancy. Many genes adopt parent-of-origin specific expression patterns in the endosperm, a phenomenon that has been termed genomic imprinting. Imprinted genes are targeted by epigenetic mechanisms acting before and after fertilization. Recent studies revealed that imprinted genes are involved in establishing seed dormancy, highlighting a new mechanism of parental control over this adaptive trait. Here, we review the regulatory mechanisms establishing genomic imprinting and their effect on seed dormancy.

Brachycephaly in French bulldogs and pugs is associated with narrow ear canals.

BACKGROUND: Brachycephalic dog breeds have multiple skull malformations which may lead to anatomical changes in the external auditory canal. It is our frequent observation that in the otoscopic examination of the external ear in these breeds we are unable to visualise the tympanic membrane as a consequence of extreme narrowing of the proximal ear canal. Additionally brachycephalic dogs reportedly are predisposed to otitis externa (OE) and otitis media. OBJECTIVES: To characterizse the transition of the cartilaginous ear canal to the bony meatus acusticus externus using computed tomography (CT) and to investigate a possible association with OE in brachycephalic dogs. MATERIALS AND METHODS: Seventy-five client-owned dogs [pugs (n = 20), French bulldogs (n = 55)] were included and assessed for OE using an owner questionnaire and otoscopic and cytological examinations. In dorsal plane CT scans, the diameter of the porus acusticus externus was measured using novel methodology. The results were compared with a normocephalic control group without preexisting otological disorders. RESULTS: Brachycephalic dogs had a significantly smaller porus acusticus externus diameter (2.6 mm) than normocephalic dogs (5.0 mm). Of the brachycephalic dogs, 32% had OE yet this was not statistically significantly related to the diameter of the porus acusticus externus. Middle ear effusion (44%) and narrowing of the external ear canal (82.6%) were significantly more frequent in brachycephalic dogs. Only five of 150 eardrums could be visualised otoscopically. CONCLUSIONS AND CLINICAL RELEVANCE: Malformation of the porus acusticus externus causes severe stenosis of the proximal ear canal in brachycephalic dogs. A connection between stenosis of the external auditory canal and OE could not be confirmed.

Contexte - Les races de chiens brachycéphales présentent de multiples malformations crâniennes qui peuvent entraîner des modifications anatomiques du conduit auditif externe. Nous observons fréquemment que lors de l'examen otoscopique de l'oreille externe chez ces races, nous sommes incapables de visualiser la membrane tympanique en raison d'un rétrécissement extrême du conduit auditif proximal. De plus, les chiens brachycéphales seraient prédisposés à l'otite externe (OE) et à l'otite moyenne. Objectifs - Caractériser la transition du conduit auditif cartilagineux au méat osseux externe par tomodensitométrie (TDM) et étudier une éventuelle association avec l'OE chez les chiens brachycéphales. Matériels et méthodes - Soixante-quinze chiens de propriétaires [carlins (n = 20), bouledogues français (n = 55)] ont été inclus et évalués pour l'OE à l'aide d'un questionnaire du propriétaire et d'examens otoscopiques et cytologiques. Dans les tomodensitogrammes du plan dorsal, le diamètre du porus acusticus externus a été mesuré à l'aide d'une nouvelle méthodologie. Les résultats ont été comparés à un groupe témoin normocéphale sans trouble otologique préexistant. Résultats - Les chiens brachycéphales avaient un diamètre de porus acusticus externus significativement plus petit (2,6 mm) que les chiens normocéphales (5,0 mm). Parmi les chiens brachycéphales, 32 % avaient une OE, mais cela n'était pas statistiquement lié de manière significative au diamètre du porus acusticus externus. L'épanchement de l'oreille moyenne (44 %) et le rétrécissement du conduit auditif externe (82,6 %) étaient significativement plus fréquents chez les chiens brachycéphales. Seuls cinq des 150 tympans ont pu être visualisés par otoscopie. Conclusions et pertinence clinique - La malformation du porus acusticus externus provoque une sténose sévère du conduit auditif proximal chez le chien brachycéphale. Un lien entre la sténose du conduit auditif externe et l'OE n'a pas pu être confirmé.

Contexto - Raças de cães braquicefálicos apresentam múltiplas malformações cranianas que podem levar a alterações anatômicas no conduto auditivo externo. Frequentemente, observamos que na avaliação otoscópica da orelha externa destas raças não conseguimos visualizar a membrana timpânica como uma consequência do estreitamento extremo do conduto auditivo proximal. Além disso, cães braquicefálicos são sabidamente predispostos à otite externa (OE) e otite média. Objetivos - Caracterizar a transição do conduto auditivo cartilaginoso para o meato acústico ósseo externo utilizando tomografia computadorizada (TC) e investigar uma possível associação com OE em cães braquicefálicos. Materiais e métodos - Setenta cães de clientes [pugs (n=20), buldogues franceses (n = 55)] foram inclusos e avaliados para OE utilizando um questionário para os proprietários, citologia e otoscopia. Nas TCs em plano dorsal, o diâmetro do poro acústico externo foi mensurado utilizando uma metodologia nova. Os resultados foram comparados com um grupo de cães normocefálicos controle sem alterações otológicas pré-existentes. Resultados - Os cães braquicefálicos apresentaram o diâmetro do poro acústico externo (2,6mm) significativamente menor que os cães normocefálicos (5,0mm). Dos cães braquicefálicos, 32% apresentavam OE, mas não houve relação significativa com o diâmetro do poro acústico externo. Presença de efusão na orelha média (44%) e estreitamento do conduto auditivo externo (82,6%) foram significativamente mais frequentes em cães braquicefálicos. Apenas cinco de 150 tímpanos puderam ser visualizados por otoscopia. Conclusões e relevância clínica - Malformações do poro acústico externo causa estenose grave do conduto auditivo proximal em cães braquicefálicos. Não foi possível comprovar a conexão entre a estenose do conduto auditivo externo e OE.

Introducción- las razas de perros braquicéfalos tienen múltiples malformaciones craneales que pueden provocar cambios anatómicos en el conducto auditivo externo. Con frecuencia observamos que en el examen otoscópico del oído externo en estas razas no podemos visualizar la membrana timpánica como consecuencia del estrechamiento extremo del conducto auditivo proximal. Además, según la literatura al respecto, los perros braquicéfalos están predispuestos a padecer otitis externa (OE) y otitis media. Objetivos - Caracterizar la transición del canal auditivo cartilaginoso al meato acusticus externus óseo mediante tomografía computerizada (CT) e investigar una posible asociación con OE en perros braquicefálicos. Materiales y métodos- setenta y cinco perros de propietarios particulares [carlinos (n = 20), bulldogs franceses (n = 55)] fueron incluidos y evaluados por posible OE mediante un cuestionario para propietarios y exámenes otoscópicos y citológicos. En tomografías computerizadas del plano dorsal, el diámetro del porus acusticus externus se midió utilizando una metodología novedosa. Los resultados se compararon con un grupo control normocefálico sin trastornos otológicos preexistentes. Resultados- los perros braquicefálicos tenían un diámetro del porus acusticus externus significativamente más pequeño (2,6 mm) que los perros normocefálicos (5,0 mm). De los perros braquicefálicos, el 32 % tenía OE, pero esto no se relacionó estadísticamente de manera significativa con el diámetro del porus acusticus externus. La efusión del oído medio (44 %) y el estrechamiento del conducto auditivo externo (82,6 %) fueron significativamente más frecuentes en perros braquicefálicos. Solo cinco de 150 tímpanos pudieron visualizarse por otoscopia. Conclusiones y relevancia clínica - La malformación del porus acusticus externus causa estenosis severa del canal auditivo proximal en perros braquicefálicos. No se pudo confirmar una conexión entre la estenosis del conducto auditivo externo y la OE.

3T MRI characteristics of the palatine tonsil in brachycephalic dogs.

BACKGROUND: Investigation of the MRI characteristics of the palatine tonsil in brachycephalic dogs in 3T high-field system. METHODS: Eighty-five brachycephalic dogs and 37 normocephalic dogs were divided into five groups: group 1 French bulldogs (FBs) with neurological clinical signs (n = 37), group 2 FBs with brachycephalic obstructive airway syndrome (BOAS) (n = 22), group 3 pugs with neurological clinical signs (n = 17), group 4 pugs with BOAS (n = 9) and group 5 normocephalic dogs (n = 37). Cross-sectional area and volume measurements were performed, and tonsillar margination and contour, shape, signal intensity and homogeneity/heterogeneity of the palatine tonsils were evaluated and compared. RESULTS: Cross-sectional area and volume measurements of the tonsils showed no significant differences between brachycephalic and normocephalic dogs with the exception of the dogs of group 2 (FB BOAS), which showed relatively high volume and large cross-sectional area in comparison to other groups. In 87% of the brachycephalic animals, the tonsils were well defined. A smooth contour was detectable in 91.8% and a rounded shape in 94.7% of brachycephalic dogs. Signal intensity was assessed as hyperintense in relation to the musculature and iso- to hyperintense to the soft palate. Heterogeneous appearance was described in 86.9% of the brachycephalic animals. CONCLUSIONS: The MRI characteristics of the tonsils of brachycephalic dogs do not differ considerably from those of normocephalic dogs. In FBs with distinct clinical signs of obstructive airway syndrome, increase in cross-sectional area and volume of the tonsils was detected.

MRI diagnosis of spontaneous intraventricular tension-pneumocephalus in a 10-month-old male Saarloos Wolfdog.

A 10-month-old male Saarloos Wolfdog was presented with a history of multiple neurologic deficits that had acutely progressed. Neurologic examination findings localized signs to the cerebrum and brainstem. Magnetic resonance imaging revealed markedly enlarged and gas-filled lateral ventricles with a mass effect leading to cerebellar herniation. A right-sided defect of the cribriform plate with a dysplastic ethmoturbinate was identified as the inlet of air and origin of the intraventricular tension pneumocephalus. Surgical findings were consistent with a ruptured, congenital, nasal meningocele.

Bypassing reproductive barriers in hybrid seeds using chemically induced epimutagenesis.

The triploid block, which prevents interploidy hybridizations in flowering plants, is characterized by a failure in endosperm development, arrest in embryogenesis, and seed collapse. Many genetic components of triploid seed lethality have been successfully identified in the model plant Arabidopsis thaliana, most notably the paternally expressed genes (PEGs), which are upregulated in tetraploid endosperm with paternal excess. Previous studies have shown that the paternal epigenome is a key determinant of the triploid block response, as the loss of DNA methylation in diploid pollen suppresses the triploid block almost completely. Here, we demonstrate that triploid seed collapse is bypassed in Arabidopsis plants treated with the DNA methyltransferase inhibitor 5-Azacytidine during seed germination and early growth. We identified strong suppressor lines showing stable transgenerational inheritance of hypomethylation in the CG context, as well as normalized expression of PEGs in triploid seeds. Importantly, differentially methylated loci segregate in the progeny of "epimutagenized" plants, which may allow epialleles involved in the triploid block response to be identified in future studies. Finally, we demonstrate that chemically induced epimutagenesis facilitates hybridization between different Capsella species, thus potentially emerging as a strategy for producing triploids and interspecific hybrids with high agronomic interest.

The miRNome function transitions from regulating developmental genes to transposable elements during pollen maturation.

Animal and plant microRNAs (miRNAs) are essential for the spatio-temporal regulation of development. Together with this role, plant miRNAs have been proposed to target transposable elements (TEs) and stimulate the production of epigenetically active small interfering RNAs. This activity is evident in the plant male gamete containing structure, the male gametophyte or pollen grain. How the dual role of plant miRNAs, regulating both genes and TEs, is integrated during pollen development and which mRNAs are regulated by miRNAs in this cell type at a genome-wide scale are unknown. Here, we provide a detailed analysis of miRNA dynamics and activity during pollen development in Arabidopsis thaliana using small RNA and degradome parallel analysis of RNA end high-throughput sequencing. Furthermore, we uncover miRNAs loaded into the two main active Argonaute (AGO) proteins in the uninuclear and mature pollen grain, AGO1 and AGO5. Our results indicate that the developmental progression from microspore to mature pollen grain is characterized by a transition from miRNAs targeting developmental genes to miRNAs regulating TE activity.

Endosperm Evolution by Duplicated and Neofunctionalized Type I MADS-Box Transcription Factors.

MADS-box transcription factors (TFs) are present in nearly all major eukaryotic groups. They are divided into Type I and Type II that differ in domain structure, functional roles, and rates of evolution. In flowering plants, major evolutionary innovations like flowers, ovules, and fruits have been closely connected to Type II MADS-box TFs. The role of Type I MADS-box TFs in angiosperm evolution remains to be identified. Here, we show that the formation of angiosperm-specific Type I MADS-box clades of Mγ and Mγ-interacting Mα genes (Mα*) can be tracked back to the ancestor of all angiosperms. Angiosperm-specific Mγ and Mα* genes were preferentially expressed in the endosperm, consistent with their proposed function as heterodimers in the angiosperm-specific embryo nourishing endosperm tissue. We propose that duplication and diversification of Type I MADS genes underpin the evolution of the endosperm, a developmental innovation closely connected to the origin and success of angiosperms.

H2A ubiquitination is essential for Polycomb Repressive Complex 1-mediated gene regulation in Marchantia polymorpha.

BACKGROUND: Polycomb repressive complex 1 (PRC1) and PRC2 are chromatin regulators maintaining transcriptional repression. The deposition of H3 lysine 27 tri-methylation (H3K27me3) by PRC2 is known to be required for transcriptional repression, whereas the contribution of H2A ubiquitination (H2Aub) in the Polycomb repressive system remains unclear in plants. RESULTS: We directly test the requirement of H2Aub for gene regulation in Marchantia polymorpha by generating point mutations in H2A that prevent ubiquitination by PRC1. These mutants show reduced H3K27me3 levels on the same target sites as mutants defective in PRC1 subunits MpBMI1 and the homolog MpBMI1L, revealing that PRC1-catalyzed H2Aub is essential for Polycomb system function. Furthermore, by comparing transcriptome data between mutants in MpH2A and MpBMI1/1L, we demonstrate that H2Aub contributes to the PRC1-mediated transcriptional level of genes and transposable elements. CONCLUSION: Together, our data demonstrates that H2Aub plays a direct role in H3K27me3 deposition and is required for PRC1-mediated transcriptional changes in both genes and transposable elements in Marchantia.

Combinations of maternal-specific repressive epigenetic marks in the endosperm control seed dormancy.

Polycomb Repressive Complex 2 (PRC2)-mediated trimethylation of histone H3 on lysine 27 (H3K27me3) and methylation of histone 3 on lysine 9 (H3K9me) are two repressive epigenetic modifications that are typically localized in distinct regions of the genome. For reasons unknown, however, they co-occur in some organisms and special tissue types. In this study, we show that maternal alleles marked by H3K27me3 in the Arabidopsis endosperm were targeted by the H3K27me3 demethylase REF6 and became activated during germination. In contrast, maternal alleles marked by H3K27me3, H3K9me2, and CHG methylation (CHGm) are likely to be protected from REF6 targeting and remained silenced. Our study unveils that combinations of different repressive epigenetic modifications time a key adaptive trait by modulating access of REF6.

Transgenerational effect of mutants in the RNA-directed DNA methylation pathway on the triploid block in Arabidopsis.

BACKGROUND: Hybridization of plants that differ in number of chromosome sets (ploidy) frequently causes endosperm failure and seed arrest, a phenomenon referred to as triploid block. In Arabidopsis, loss of function of NRPD1, encoding the largest subunit of the plant-specific RNA polymerase IV (Pol IV), can suppress the triploid block. Pol IV generates short RNAs required to guide de novo methylation in the RNA-directed DNA methylation (RdDM) pathway. Recent work suggests that suppression of the triploid block by mutants in RdDM components differs, depending on whether the diploid pollen is derived from tetraploid plants or from the omission in second division 1 (osd1) mutant. This study aims to understand this difference. RESULTS: In this study, we find that the ability of mutants in the RdDM pathway to suppress the triploid block depends on their degree of inbreeding. While first homozygous generation mutants in RdDM components NRPD1, RDR2, NRPE1, and DRM2 have weak or no ability to rescue the triploid block, they are able to suppress the triploid block with successive generations of inbreeding. Inbreeding of nrpd1 was connected with a transgenerational loss of non-CG DNA methylation on sites jointly regulated by CHROMOMETHYLASES 2 and 3. CONCLUSIONS: Our data reveal that loss of RdDM function differs in its effect in early and late generations, which has important implications when interpreting the effect of RdDM mutants.

Postzygotic reproductive isolation established in the endosperm: mechanisms, drivers and relevance.

The endosperm is a developmental innovation of angiosperms that supports embryo growth and germination. Aside from this essential reproductive function, the endosperm fuels angiosperm evolution by rapidly establishing reproductive barriers between incipient species. Specifically, the endosperm prevents hybridization of newly formed polyploids with their non-polyploid progenitors, a phenomenon termed the triploid block. Furthermore, recently diverged diploid species are frequently reproductively isolated by endosperm-based hybridization barriers. Current genetic approaches have revealed a prominent role for epigenetic processes establishing these barriers. In particular, imprinted genes, which are expressed in a parent-of-origin-specific manner, underpin the interploidy barrier in the model species Arabidopsis. We will discuss the mechanisms establishing hybridization barriers in the endosperm, the driving forces for these barriers and their impact for angiosperm evolution. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

On the origin of the widespread self-compatible allotetraploid Capsella bursa-pastoris (Brassicaceae).

Polyploidy, or whole-genome duplication, is a common speciation mechanism in plants. An important barrier to polyploid establishment is a lack of compatible mates. Because self-compatibility alleviates this problem, it has long been hypothesized that there should be an association between polyploidy and self-compatibility (SC), but empirical support for this prediction is mixed. Here, we investigate whether the molecular makeup of the Brassicaceae self-incompatibility (SI) system, and specifically dominance relationships among S-haplotypes mediated by small RNAs, could facilitate loss of SI in allopolyploid crucifers. We focus on the allotetraploid species Capsella bursa-pastoris, which formed ~300 kya by hybridization and whole-genome duplication involving progenitors from the lineages of Capsella orientalis and Capsella grandiflora. We conduct targeted long-read sequencing to assemble and analyze eight full-length S-locus haplotypes, representing both homeologous subgenomes of C. bursa-pastoris. We further analyze small RNA (sRNA) sequencing data from flower buds to identify candidate dominance modifiers. We find that C. orientalis-derived S-haplotypes of C. bursa-pastoris harbor truncated versions of the male SI specificity gene SCR and express a conserved sRNA-based candidate dominance modifier with a target in the C. grandiflora-derived S-haplotype. These results suggest that pollen-level dominance may have facilitated loss of SI in C. bursa-pastoris. Finally, we demonstrate that spontaneous somatic tetraploidization after a wide cross between C. orientalis and C. grandiflora can result in production of self-compatible tetraploid offspring. We discuss the implications of this finding on the mode of formation of this widespread weed.