Speciation and development

Embryonic Lethality, Juvenile Growth Variation, and Adult Sterility Correlate With Phylogenetic Distance of Danionin Hybrids

Hybrid incompatibility, which plays a pivotal role in speciation, is expected to correlate with greater phylogenetic distance. Here, we investigate the fitness of interspecies hybrids within the Danionin subfamily, which includes the model species, Danio rerio, and its relatives - Danio kyathit, Danio albolineatus, Danio margaritatus, and Devario aequipinnatus. We generated hybrids through in vitro fertilization, using Danio rerio as the maternal species, with normal fertilization rates showing no incompatibilities in sperm-egg interactions within these two genera. Generally, all hybrids exhibit normal patterns and timelines in early developmental transitions, from cleavage stages to the initiation of epiboly, although inter-genera Danio-Devario hybrids subsequently exhibit fully penetrant embryonic lethality. Intra-genus Danio hybrids, on the other hand, can survive through embryogenesis and into adulthood. However, rates of survival during these stages diminish according to phylogenetic distance, with increasing early lethality in hybrids from more distantly related species. Additionally, Danio hybrids exhibit increased growth rate variability during juvenile stages. All Danio hybrids have reduced testes sizes, sperm counts, and sperm viabilities, with sperm displaying defects in flagellum formation and integrity. Adult male intra-genus hybrids are invariably sterile, except in the case of Danio rerio hybrids with the closely related Danio kyathit, which produced a backcrossed F2 generation that did not survive juvenile stages. Our studies highlight a loss of hybrid compatibility at various life stages in the Danio and Devario genera, based on deleterious effects and reduced developmental robustness, emphasizing a correlation between the severity of incompatibility outcomes and the degree of phylogenetic relatedness.

Acquisition of polymorphism in the chordate doliolids

In polymorphic organisms, a single genome is deployed to program numerous, morphologically distinct body plans within a colony. This complex life history trait has evolved independently within a limited subset of animal taxa. Reconstructing the underlying genetic, cellular, and developmental changes that drove the emergence of polymorphic colonies represents a promising avenue for exploring diversifying selection and resulting impacts on developmental gene regulatory networks. Doliolids are the only polymorphic chordate, deploying a single genome to program distinct morphs specialized for locomotion, feeding, asexual, or sexual reproduction. In this review, we provide a detailed summary of doliolid anatomy, development, taxonomy, ecology, life history, and the cellular basis for doliolid polymorphism. In order to frame the potential evolutionary and developmental insights that could be gained by studying doliolids, we provide a broader overview of polymorphism. We then discuss how comparative studies of polymorphic cnidarians have begun to illuminate the genetic basis of this unusual and complex life history strategy. We then provide a summary of life history divergence in the chordates, particularly among doliolids and their polymorphic cousins, the salps and pyrosomes.

Beyond Haldane's rule: Sex-biased hybrid dysfunction for all modes of sex determination

Haldane's rule occupies a special place in biology as one of the few 'rules' of speciation, with empirical support from hundreds of species. And yet, its classic purview is restricted taxonomically to the subset of organisms with heteromorphic sex chromosomes. I propose explicit acknowledgement of generalized hypotheses about Haldane's rule that frame sex bias in hybrid dysfunction broadly and irrespective of the sexual system. The consensus view of classic Haldane's rule holds that sex-biased hybrid dysfunction across taxa is a composite phenomenon that requires explanations from multiple causes. Testing of the multiple alternative hypotheses for Haldane's rule is, in many cases, applicable to taxa with homomorphic sex chromosomes, environmental sex determination, haplodiploidy, and hermaphroditism. Integration of a variety of biological phenomena about hybrids across diverse sexual systems, beyond classic Haldane's rule, will help to derive a more general understanding of the contributing forces and mechanisms that lead to predictable sex biases in evolutionary divergence and speciation.

Exploring the patterns of evolution: Core thoughts and focus on the saltational model

The Modern Synthesis, a pillar in biological thought, united Darwin's species origin concepts with Mendel's laws of character heredity, providing a comprehensive understanding of evolution within species. Highlighting phenotypic variation and natural selection, it elucidated the environment's role as a selective force, shaping populations over time. This framework integrated additional mechanisms, including genetic drift, random mutations, and gene flow, predicting their cumulative effects on microevolution and the emergence of new species. Beyond the Modern Synthesis, the Extended Evolutionary Synthesis expands perspectives by recognizing the role of developmental plasticity, non-genetic inheritance, and epigenetics. We suggest that these aspects coexist in the plant evolutionary process; in this context, we focus on the saltational model, emphasizing how saltation events, such as dichotomous saltation, chromosomal mutations, epigenetic phenomena, and polyploidy, contribute to rapid evolutionary changes. The saltational model proposes that certain evolutionary changes, such as the rise of new species, may result suddenly from single macromutations rather than from gradual changes in DNA sequences and allele frequencies within a species over time. These events, observed in domesticated and wild higher plants, provide well-defined mechanistic bases, revealing their profound impact on plant diversity and rapid evolutionary events. Notably, next-generation sequencing exposes the likely crucial role of allopolyploidy and autopolyploidy (saltational events) in generating new plant species, each characterized by distinct chromosomal complements. In conclusion, through this review, we offer a thorough exploration of the ongoing dissertation on the saltational model, elucidating its implications for our understanding of plant evolutionary processes and paving the way for continued research in this intriguing field.