Aberrant Classopollis pollen reveals evidence for unreduced (2 n ) pollen in the conifer family Cheirolepidiaceae during the Triassic–Jurassic transition

Abstract

Polyploidy (or whole-genome doubling) is a key mechanism for plant speciation leading to new evolutionary lineages. Several lines of evidence show that most species among flowering plants had polyploidy ancestry, but it is virtually unknown for conifers. Here, we study variability in pollen tetrad morphology and the size of the conifer pollen type Classopollis extracted from sediments of the Triassic-Jurassic transition, 200 Ma. Classopollis producing Cheirolepidiaceae were one of the most dominant and diverse groups of conifers during the Mesozoic. We show that aberrant pollen Classopollis tetrads, triads and dyads, and the large variation in pollen size indicates the presence of unreduced (2n) pollen, which is one of the main mechanisms in modern polyploid formation. Polyploid speciation may explain the high variability of growth forms and adaptation of these conifers to different environments and their resistance to extreme growth conditions. We suggest that polyploidy may have also reduced the extinction risk of these conifers during the End-Triassic biotic crisis.

Analysis of 41 plant genomes supports a wave of successful genome duplications in association with the Cretaceous–Paleogene boundary

“…Similar observations exist in interspecific Brassica hybrids subject to cold stress (Mason et al 2011), while most hybrids already exhibit increased levels of unreduced gamete formation (Ramsey and Schemske 1998). Recent evidence supports that environmental stress and/or fluctuations could also have increased unreduced gamete levels at previous large-scale extinctions, as demonstrated by the increased number of unreduced fossil pollen found in the now extinct conifer family Cheirolepidiaceae at the Triassic-Jurassic transition 201.3 mya (Kurschner et al 2013). Abnormal gymnosperm pollen (Foster and Afonin 2005) and lycophyte spores (Visscher et al 2004) have also been reported at the Permian-Triassic transition 252.3 mya (Shen et al 2011).…”

How much do we know about the frequency of hybridisation and polyploidy in the Mediterranean region?

“…altitudinal gradients), as well as large‐scale climate changes (as shown in Kürschner et al . ; Vanneste et al . ) could substantially alter the dynamics of polyploid evolution, or at least fuel opportunities for establishment through the recurrent formation of new entities.…”

Paleobotany and Global Change: Important Lessons for Species to Biomes from Vegetation Responses to Past Global Change

“…In other regions of the supercontinent Pangea and in East Greenland, the ETE is marked by a geologically transient ecological dominance of ferns and fern allies (87) belonging to extant fern families, many of which today have exclusively tropical and subtropical biogeographical distributions. Thermophilic conifers belonging to the extinct Cheirolepidiaceae family became dominant in the continental interiors in many global localities (22), perhaps aided by widespread fire disturbance (15) and their likely capacity for whole-genome duplication (81), which is generally rare in gymnosperms. Therefore, the pattern of global vegetation responses under super-elevated CO 2 is characterized by a loss of evenness (101) and local-to-regional-scale turnover (22, 87, 90, 98), but remarkably little global extinction at higher taxonomic ranks (30,166).…”