Unrestricted gene flow between two subspecies of translocated brushtail possums (Trichosurus vulpecula) in Aotearoa New Zealand

Two lineages of brushtail possums (Trichosurus vulpecula) were historically introduced to Aotearoa New Zealand, and these two subspecies have different phenotypic forms. Despite over 100 years of potential interbreeding, they appear to retain morphological differences, which may indicate reproductive isolation. We examined this using population samples from a confined landscape and scored each specimen for phenotype using a number of fur colour traits. This resulted in a bimodal trait distribution expected for segregated grey and black lineages. We also sought evidence for genetic partitioning based on spatial and temporal effects. Genetic structure and rates of genetic mixing were determined using seven neutral, species-specific nuclear microsatellite markers and mitochondrial DNA control region sequence. Genotype analyses indicated high levels of variation and mtDNA sequences formed two major haplogroups. Pairwise tests for population differentiation of these markers found no evidence of subdivision, indicating that these brushtail possums behave as a single randomly mating unit. Despite maintenance of two main colour phenotypes with relatively few intermediates, previous inference of assortative mating and anecdotes of distinct races, our data indicate that New Zealand brushtail possums can freely interbreed, and that in some locations they have formed completely mixed populations where neutral genetic markers are unrelated to phenotype. This has implications for effective pest management towards eradication.

Numerical Simulation of the Complex Impact Behavior of Float-Over Deck Installation Based on an Efficient Two-Body Heaving Impact Model

Float-over deck installation involves multi-body interactions under the wave excitations, such as the nonlinear impacts between the barge and deck via the Deck Support Units (DSUs) and between deck and substructure via the Leg Mating Unit (LMUs). These nonlinear impacts can only be analysed in the time-domain. This paper develops an efficient two-body heaving impact model based on the Cummins equation to study the nonlinear impact behaviour of float-over deck installation. In this model, the convolution term of the Cummins equation is replaced by state-space model such that the efficiency of time-domain modelling can be greatly enhanced. Both the DSUs and LMUs, serving as the shock absorbing devices, are modelled as linear compression-only springs with limited carrying capacity. When the carrying capacity of DSU and LMU is reached, direct contact between deck and barge and between deck and substructure are also modelled by using two harder compression-only springs. The established model is applied to study the nonlinear dynamics of the float-over system during the mating stage that is divided into five stages according to the percentage of deck load transferred to the substructure. Bifurcation diagram is also applied to demonstrate the nonlinear behaviour associated with the deck and barge subjected to LMU and DSU impacts. Hard impacts, namely the direct impacts between the deck and barge and between the deck and substructure, together with high frequency vibrations are found to occur at the start and end of the mating stage. The motion pattern of the deck evolving from periodic motions into chaotic motions is identified. In addition, the period-doubling phenomenon is also observed.

Float-Over Installation Analysis of Semisubmersible Production Platform Topside

Compared with split lifting by floating crane, the float-over is less time-consuming and has larger lifting capacity, especially suitable for large and medium sized offshore platforms. Float-over installation process of a deepwater semisubmersible platform topside is analyzed in this paper. And the multi-floating coupling calculation is carried out in the typical loading conditions of mating process between topside and column top structure. The motion response of leg mating unit in position of barge is received. The calculation results show that the float movement condition meets float-over installation requirements of topside. The analysis can provide a reference for the project of the float-over installation in the future.

Design of Leg Mating Unit for Float-Over Installation of Decks

The Leg Mating Unit (LMU) is a critical component in ensuring safe method of installing topsides of offshore oil and gas platforms by the float-over method. Traditionally, topsides are lifted onto the substructure (e.g. jacket) using heavy lift crane vessels. However, the ‘lift’ method of installation is constrained by the availability of a limited number of heavy lift vessels in the region, with high day rates. As an alternative to modular installation with light crane vessels, float-over installation enables installation of a single pre-commissioned integrated deck, minimizing offshore hook-up time and cost. Further, float-over method is particularly suited to shallow water depth locations, remote locations (with no access to crane vessels). In a float-over installation, the deck is transported on a cargo barge to the pre-installed substructure location. The barge is guided into the jacket slot and positioned so that the stabbing cone on each leg is aligned with the corresponding jacket leg. The barge is then ballasted down (aided by the falling tide) so that the topside load is transferred from the barge to the jacket. Once the load is transferred and sufficient clearance is achieved between the deck structure and barge support structure, the barge is withdrawn from the slot. The transfer of load is the crucial step of a float-over installation and should occur in a controlled manner under the dynamic influence of environmental forces. This smooth load transfer is achieved using LMU’s. LMU’s are customized leg and deck mating units, used to dampen the impact loads generated during the mating process. They consist of steel structures with elastomer elements and are designed to perform three primary functions, absorb shocks, limit hammering onto the structures and provide defined stiffness between deck and sub-structure. The objective of this paper is to outline the design philosophy of a LMU and address the behavior of the LMU under the combination of vertical and horizontal loads during the mating process. The paper also recommends guidelines on the selection of elastomer stiffness based on load-displacement relationship. The LMU is analysed in ABAQUS, a commercially available finite element (FE) analysis package considering a non-linear time-domain analysis. The results from the FE analysis are compared with the qualification tests for the elastomer and LMU assembly performed on-site to establish correlation.

On the Extinction of a Class of Population-Size-Dependent Bisexual Branching Processes

In this paper, we study a class of bisexual Galton-Watson branching processes in which the law of offspring distribution is dependent on the population size. Under a suitable condition on the offspring distribution, we prove that the limit of mean growth-rate per mating unit exists. Based on this limit, we give a criterion to identify whether the process admits ultimate extinction with probability one.

On the Extinction of a Class of Population-Size-Dependent Bisexual Branching Processes

In this paper, we study a class of bisexual Galton-Watson branching processes in which the law of offspring distribution is dependent on the population size. Under a suitable condition on the offspring distribution, we prove that the limit of mean growth-rate per mating unit exists. Based on this limit, we give a criterion to identify whether the process admits ultimate extinction with probability one.

JOINT FREQUENCIES OF ALLELES DETERMINED BY SEPARATE FORMULATIONS FOR THE MATING AND MUTATION SYSTEMS

ABSTRACT A method to calculate joint gene frequencies, which are the probabilities that two neutral genes taken at random from a population have certain allelic states, is developed taking into account the effects of the mating system and the mutation scheme. We assume that the mutation rates are constant in the population and that the mating system does not depend on allelic states. Under either–the condition that mutation rates are symmetric or that the mating unit is large and the mutation rate is small–the general formula is represented by two terms, one for the mating system and the other for the mutation scheme. The term for the mating system is expressed using the coancestry coefficient in the infinite allele model, and the term for the mutation scheme is a function of the eigenvalues and the eigenvectors of the mutation matrix. Several examples are presented as applications of the method, including homozygosity in a stepping-stone model with a symmetric mutation scheme.

A note on extinction criteria for bisexual Galton-Watson processes

This note deals with extinction criteria for bisexual Galton–Watson processes with arbitrary mating functions in terms of the averaged reproduction mean per mating unit. It gives a satisfactory answer to a question put forward by Hull (1982).

A note on extinction criteria for bisexual Galton-Watson processes

This note deals with extinction criteria for bisexual Galton–Watson processes with arbitrary mating functions in terms of the averaged reproduction mean per mating unit. It gives a satisfactory answer to a question put forward by Hull (1982).