Evolution and Morphological Shape Ontogeny of the Brown ticks (Acari: Ixodida: Ixodidae: Rhipicephalus)
Date
2021-04
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Rhipicephalus constitute a species-diverse genus of mammal ectoparasites mainly distributed in the
Afrotropics that are characterised by generally inornate, uniform brown body colour,
short hypostome and palps, basis capituli approximately hexagonal, eyes present and male
adanal plates present. They transmit microparasites such as Rickettsia spp., Theileria spp. and
Babesia spp. to livestock and humans alike, and some inject neurotoxins during feeding that
lead to tick paralysis in livestock. This work infers the phylogeny of Rhipicephalus from
molecular lines of evidence (12S, 16S, COI and 28S-D2) and uses this as a basis to
infer aspects of their evolutionary history, ecology and evolutionary-development based on
geographic distribution data and basis capitulum shape data. Analyses included estimation of
divergence times, ancestral area optimisations, ancestral host-use optimisations in immatures and
adults, as well as estimations of ancestral climate niches. Basis capitulum morphology is
quantified to determine evolutionary- developmental modifications, which are linked to
similar patterns in overall body size. Major outcomes of studying Rhipicephalus evolution
are 1) radiations coincide with mammal evolution and dispersal, 2) host-use at immature
stages partially explain extent of geographic ranges as well as basis capitulum morphology for
boring into thick host skin, 3) evolutionary host switches were facilitated by off-host periods
and nested connections in predator-prey food webs, 4) speciation partially resulted
from niche partitioning along temperature variation gradients, which was reinforced
by interspecific competition, and 5) evolutionary-developmental modification (basis
capitulum and overall body size) resulted from responses to distinct sets of selection pressure in
on- and off-host environments taking into account one-, two- and three-host life
cycles. The persistent taxonomic problem of R. turanicus between Palearctic and
Afrotropical regions was investigated using integrated lines of evidence to test the
species boundary in an iterative framework. This revealed two distinct species in these
regions, and the Afrotropical species is described under the name R. afranicus. Another
aim of this work was to test the hypothesis of phylogenetic recapitulation in
post-embryonic stages of Rhipicephalus basis capituli, where early developmental stages
resemble ancestral adults. However, findings indicate no signal for phylogenetic
recapitulation is present, most likely due to the action of selection that shape basis capituli
over evolutionary timescales. Selection supersedes any possible background action of
condensing selection through development that would produce phylogenetic recapitulation.
As such, this work serves as a first step for investigating phylogenetic recapitulation using shape
data, and suggests future investigation should consider embryonic life stages, alternative features
under
less selection, or wider phylogenetic comparisons.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Ticks -- Phylogeny, Brown ticks -- Parasites -- Identification, Brown dog tick, Rhipicephalus -- Morphology, UCTD