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Abstract Key messagePollen tubes from closely related species and mutants lacking pollen tube MYB transcription factors are able to initiate FER/LRE-dependent synergid cell calcium oscillations. AbstractReproductive isolation leads to the evolution of new species; however, the molecular mechanisms that maintain reproductive barriers between sympatric species are not well defined. In flowering plants, sperm cells are immotile and are delivered to female gametes by the pollen grain. After landing on the stigmatic surface, the pollen grain germinates a polarized extension, the pollen tube, into floral tissue. After growing via polar extension to the female gametes and shuttling its cargo of sperm cells through its cytoplasm, the pollen tube signals its arrival and identity to synergid cells that flank the egg. If signaling is successful, the pollen tube and receptive synergid cell burst, and sperm cells are released for fusion with female gametes. To better understand cell–cell recognition during reproduction and how reproductive barriers are maintained between closely related species, pollen tube-initiated synergid cell calcium ion dynamics were examined during interspecific crosses. It was observed that interspecific pollen tubes successfully trigger synergid cell calcium oscillations—a hallmark of reproductive success—but signaling fails downstream of key signaling genes and sperm are not released. This work further defines pollen tube–synergid cell signaling as a critical block to interspecific hybridization and suggests that the FERONIA/LORELEI signaling mechanism plays multiple parallel roles during pollen tube reception.
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Hybridization, reinforcement selection, and sex-dependent reproductive character displacement of sperm and egg recognition proteins
Abstract The establishment of reproductive isolation between species via gametic incompatibility initially requires within-species variation in reproductive compatibility. We investigate how within-species variation in sperm and egg recognition proteins, potentially generated via sexual conflict, influences reproductive isolation between two partially sympatric sea urchin species; the North American west coast Mesocentrotus franciscanus and the circumpolar Strongylocentrotus droebachiensis. Barriers to hybridization are stronger when eggs are given a choice of conspecific versus heterospecific sperm and the variation in hybridization among crosses can be explained by whether the sperm or egg protein variant is ancestral or derived. Derived proteins can be recognized as different and prevent hybridization. Examination of the allele frequencies of these proteins in M. franciscanus in and out of sympatry with S. droebachiensis along the west coast of North America reveals evidence of reinforcement selection and reproductive character displacement in eggs but not sperm, which likely reflects the differential cost of hybridization for males and females.
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- Award ID(s):
- 2025492
- PAR ID:
- 10636728
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Evolution
- Volume:
- 79
- Issue:
- 8
- ISSN:
- 0014-3820
- Format(s):
- Medium: X Size: p. 1522-1532
- Size(s):
- p. 1522-1532
- Sponsoring Org:
- National Science Foundation
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