Abstract
The Chilean endemic genus Costesia belongs to the Gigaspermaceae, one of the most basal groups of arthrodontous mosses. While none of the species in this family has a peristome, earlier stages of sporophyte development often disclose its basic structure. The study of Costesia sporophytes at the early stages of development was conducted to identify possible similarities with Diphyscium, the genus sister to Gigaspermaceae plus all other arthrodontous mosses in the moss phylogenetic tree. Diphyscium shares a strongly unequal cell division pattern with the Dicranidae. In groups more closely related to Diphyscium, as it is the case of Costesia, this pattern is not known. Our study of Costesia found only irregular presence of slightly unequal cell divisions that may then be considered as a plesiomorphic state in peristomate mosses. The most frequently present pattern revealed in Costesia is common with the Polytrichaceae, a more basal moss group with nematodontous peristomes.
Highlights
The peristome is a structure at the mouth of moss capsules, which enhances the process of spore release by means of hygroscopic movements
The comparison of complete series of the transverse sections with longitudinal sections (Figures 2 and 3) ensures that the regular patterns illustrated in Figure 4 correspond to the level of the proximal part of the “peristomial zone”, the discussion on the peristomial formula for Costesia should be as important as it is for other eperistomate mosses, like Physcomitrium and Aphanorhegma (Schwartz, 1994)
The obtained observation revealed two patterns in Costesia worth discussing: (1) asymmetric divisions occur in IPL in most distal part of the amphithecium
Summary
The peristome is a structure at the mouth of moss capsules, which enhances the process of spore release by means of hygroscopic movements. Peristome Development in Costesia comprehensive analysis by Liu et al (2019) summarized molecular data and built a robust phylogenetic tree, where peristomate mosses are found in a grade of seven classes/ subclasses and two terminal clades, the subclasses Bryidae (with diplolepideous alternate peristomes) and Dicranidae (with haplolepideous peristomes). These two subclasses include no less than 90% of the contemporary species diversity. The mosses of the grade from Takakiales to Timmiales (Figure 1), have much fewer species in the modern flora
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